Ultimately I think the terminology on the wiki is slightly inaccurate, or rather they are referring to it in relation to the iterations of the universes of seasons 1-3, as those are the iterations that our main characters are from, even though they are not the prime universes, because the events of these parallel universes coming to die has happened many to infinite times before...
It's an alternate future by comparison to their timeline(s)/Universes and because it becomes alternate with the arrival of Red Peter (Blue) Olivia, and later (blue) Walter, but in terms of event history, it seems like their universe would be their prime universe and are not the same universes as the season 1-3's. But in truth we don't know if that "future" is the same as the season 4 & 5 timelines, but seems likely...
This is more of a theory through process of elimination and/or using many of the plots to take an educated guess
We come to learn by the end of season 3 that not only is there a set of inextricably linked "blue" and a "red" parallel universes, but additionally we learn a bit more through a tie-in canon comic that extends the season three finale episode (The Day We Died) titled, Peter and Machine that these universes have several iterations of them that have aready existed...
The comic, at first through a 2026 blue Walter, further explains that these universes have died over and over again, possibly infinite times, with other versions of Peter and/or The First People taking the machine parts back in time and redistributing them at various points in history, so they can be found again. Peter finally chooses balance!
Then season 4 acts as a kind of reset (the newly introduced universes are also refereed to as amber time lines, but they are really just different iterations of the red and blue universe) where the machines are mysteriously bridged in an alternate 2011 without any Peters surviving past childhood in either universe, but as this alternate blue Walter explains later, this is a timeline that somehow gets "reprinted over" (like a video cassette tape) with the main three characters from the season 1-3 timeline(s) replacing the others (amber/blue), as Red Peter re-materializes (which results in reciprocity of Amber-red Lincoln Lee's death), Blue Olivia takes over Amber-Blue Olivia conscious, and in season 5, Amber-blue Walter finally gets Blue Walter's memories from Micheal...
Observers are humans that were created in 2167. Scientists in Oslo,
Norway, were experimenting with the human brain in that year. They
discovered that they could remove areas of the brain that stimulated
aggression and other negative emotions and replace them with rational
thought. However, over time, areas of good emotions, like love, were
removed as well in favor of higher intelligence. Over the course of
many years, Observers populated the Earth. ("The Boy Must Live")
However we also learn in season 5 with the occupation of the Observers that they are created in 2167 and apparently have polluted their future of their timeline and have come to occupy 2015+; most of season 5 takes place in a amber-blue 2036.
In addition we learn a story about September trying to save his son, Micheal, and we learn that this is The Child Observer blue Olivia & gang met in the season 1 blue timeline (Inner Child)! This means that the season 4-5 time line originally existed BEFORE the season 1-3 red & blue timelines/universes, as Michael was taken from "here" to there, and back again, but now the time line is reprinted over with those versions of those characters now living here.
We might then assume that this is also the original time line of the Observers....
From the information we have, the universes seem to have kept dying around the 2026--2030's-ish+ mark, never reaching that 2167 creation date, because we only ever have "one set" of the same Observers with no alternate versions.
It would make some sense if the amber-blue time line of season 4 & 5 *was the original timeline of their creation (before it gets reprinted over), as picking the 2015+ dates to occupy it, make sense in terms of it being more personal for them and perhaps as a way to try and safeguard their creation date, which Walter eventually goes to the future and seems to alter something, in order to prevent them from occupying 2015+.