You're touching on an interesting idea; but part of the question doesn't quite make sense.
The pub is irrelevant.
I suppose the only way to verify these hypotheses is checking if somebody at the pub scene was present in a simulation.
The result of the matchmaking was not decided based on the fact that both people were at a pub at the time. The system was not comparing Amy and Frank to the other pubgoers (specifically), it was assessing their mutual compatibility.
The most important thing to take away here is that the only relevant result of the simulation was Frank and Amy's compatibility. The system (which we see end in a 99.8% match) was not in any way checking who out of the present pubgoers was most compatible with Frank (or Amy). The system was assessing Frank and Amy's romantic compatibility.
Note: It's not impossible that the system is bigger than what we saw, and e.g. ran simulations for Amy with all nearby men. However, we only see the system respond with her match % with Frank; so unless otherwise specified; I'm going to assume that the system was only calculating Frank and Amy's compatibility.
If Frank had decided to stay with "the boring one" 990 times and escape with Amy 10 times, the result would have been 1.0%, not a suggestion to date Nicola instead.
I say "the boring one" instead of Nicola, because there's no reason to assume that the same NPCs were being used for every simulation.
The simulations were heavily scripted.
Notice the intention of the simulation: counting the number of times where Frank and Amy dismissing their (logically decided) soulmate. In other words; it counts the number of times where Frank and Amy have such strong feelings for each other that they let their emotions overtake their logic.
Logically, they are repeatedly told that "the system works", and they seem to both believe that they will eventually find a true soulmate at the end of the program.
However, when Frank and Amy meet each other, 998 times out of 100, they feel so strongly about each other that they are convinced that the system is wrong and they are right.
In other words, the system counts the number of times Frank and Amy both feel absolutely certain about being with each other, even beyond what a seemingly perfect matchmaking algorithm is telling them.
That is often how true love gets portrayed (Romeo and Juliet is a perfect example: lovers caught in a situation that tries to force them apart).
On to the question: are the other people real people or not?
We can't really answer this conclusively, as the show never told us. But we can look at the options and see which makes more sense.
Option 1 - The people are artifically constructed.
This seems like a bad setup.
It will be hard for a system to procedurally generate accurate personalities. Notice that in this futuristic world, we still haven't seen any AI that is capable of passing a Turing test. Humans are still superior to machines, and are able to spot artificial behavior (because it's notably different from human behavior).
Applying this, that would also mean that the matchmaking system might be capable of creating artificial people, but the generated personalities would not be as good as "the real deal" (real people), simply because machines aren't at that point yet.
This is also indirectly confirmed by episodes such as White Christmas. The woman's consciousness is transferred into a cookie, intended to run the real woman's household. It requires Jon Hamm to train/torture the cookie into being obedient, which he is apparently exceptionally good at.
If technology was at a point where articifial personalities could be created, there would be no point to trying to coerce a "real human" cookie into doing its work. Instead, they'd be able to tweak the cookie so that it is obedient from the get go.
Also notice that Jon Hamm needed to have the other guy confess to his crimes. This suggests that they couldn't just read his memories from his cookie, but needed his personality to retrieve his memories for him. In other words: they can clone a consciousness and control its new environment, but they cannot control or even read a consciousness directly.
Secondly, it would also be a reality failure. The system is trying to find your best match from a pool of real people.
- Inferior artificial personalities would make it more likely for Frank to pick Amy as she is "the only person who feels real".
- Using inferior personalities would mean the outcome of the simulation doesn't actually compare Frank and Amy's match ratio to other real people; which makes the result comparatively meaningless.
In my opinion, it only makes sense for the people to have been (based on) real people.
Option 2 - The other people are real, and randomly selected.
This is, I think, the option you're erring towards. Let's say (for example's sake) that there were exactly 1,000 people living in that simulated community.
Following this option, the system then put Frank, Amy and 998 other customers together, and had them meet each other. Frank and Amy were likely scripted to run into each other a few times, but other people were assigned seemingly randomly.
Due to the nature of "passing" the test (by eloping); Frank and Amy are scripted to get matched to unknown strangers, and "coincidentally" run into each other before it happens.
This makes more sense. However, I suspect that the system was much better designed than just a random selection. More elaboration in option 3.
Option 3 - The other people are real, and chose for particular reasons.
This seems much closer to what we see happen in the simulation we get to see.
- Frank is initially apprehensive of coming across as too eager. He seems to take a waiting approach, until the system reveals his soulmate. Nicola seems to be a perfect pick to teach Frank a much needed life lesson: she never wishes to learn anything about Frank (she is set in her ways), and subsequently becomes so boring that Frank gets excited about meeting literally anyone else. It teaches Frank to value other people, and to realize that he shouldn't just settle for anyone.
- Amy, on the other hand, seems to be relatively open to any relationship that comes her way. She's not particularly picky about men (note that she did respond positively to the next guy's looks, but she didn't particularly shoot down Frank by how he looks, and she was capable of looking at his character instead). She also got a very applicable progression to teach her a much needed life lesson: a string of short and empty relationships, while initially thrilling her, eventually leaves her feeling empty inside. It makes her value more than just a superficial match, and it can't just be coincidence that Frank makes much more sense as a partner in the long term, as opposed to the short term (due to his apprehensive behavior in the short term).
- During Frank and Amy's second relationship, everything works out well, until Frank makes another relationship foul: breaking a promise. He is subsequently punished. This opens an interesting avenue for both Frank and Amy: do they resort to blaming each other? Are they capable of working through a negative event? Does Amy still want Frank after he messes up her (and his) happiness? All of these questions are incredibly relevant to considering someone as a romantic match (imagine if new couples could see how they'd behave after a long time together, when things get hard and someone is to blame); and I don't believe that this event was randomly stumbled upon.
- The penalty that Frank receives for looking at the expiry date seems to again be chosen very specifically: it focuses him to reevaluate the mistake he made before.
- As an aside, also note that the system gives Frank an explanation for the penalty. Paraphrasing, it's something along the lines of "this penalty will teach you to deal with breaking trust/promises to your partner". The system knows how to punish/reward negative/positive behavior in order to gauge romantic compatibility. This very much suggests that the system is not just matching random people and seeing what happens, but is actively steering the matchmaking based on its assessment of Frank (and others).
The above bullet points, in my opinion, strongly suggest that the system was handpicking Frank and Amy's matches, in order to see if they were willing to self-improve in order to be with Amy/Frank (e.g. Frank being more proactive about showing someone he likes her, so that he doesn't leave Amy hanging like he did the first date).
In other words, after the first date, the system intentionally picked someone for Frank who he would hate, for a long time. To really drive the point home.
- That doesn't mean Frank was always going to be assigned to Nicola. Maybe the system simply randomly picked someone who fits the script's criteria; i.e. Frank could've been assigned (in different simulations) to someone else who he'd hate being in a relationship with. It makes sense, as repeatedly pairing Frank with Nicola (instead of changing her around) gives you less relevant matchmaking data.
- There's no guarantee that Frank and Amy made the same mistakes every first date. Maybe Frank was highly self-confident in another simulation, and therefore ends up on a track similar to Amy's (being assigned to hollow short flings). Maybe Frank only ends up with a Nicola (or similar person) in a given % of cases.
- Given that both Frank and Amy are new to the matchmaking system when they have their first date, it stands to reason that this is where the simulation begins; and therefore that the system decides who to pair them up with based on the flaws they exhibit during the first date.