After being offered employment, Jimmy immediately has a change of
heart and decides he doesn’t want it. My question isn’t why he had the
change of heart, and didn’t want the job; my question is why he was so
berating to the owners?
I'm combining my comments for an answer.
The "why" matters:
I know you stated that you didn't want to know "why" he did it, but IMO one has to know why in order to better answer your real question, because it is a layered scene that ties everything that has accumulated in the previous seasons together.
EW Interview with Bob Odenkirk (Warning Episode 4.05 spoilers in EW link)
And there’s that little Band-Aid box that has the coins that he found
in season 2 of Better Call Saul when he had a flashback to his youth
and his dad’s store. He told the story of his father and how he
perceived his dad as a person who was taken advantage of too easily
and he really despised his father for that, which makes him feel bad.
I think that has to do with that Neff episode [“Breathe”] when he went
to the copy shop and he called them losers because they believed his
sales pitch. It made him angry because in his mind, it was such a
bald-faced show that he was putting on and how they could not see that
just pissed him off. He just couldn’t take it, and I think he’s really
talking to his dad there and saying, “You’re a sucker, and I couldn’t
stand to be around you.”
The desegregation of the men, specifically when he calls them "suckers", is reference calling back to Jimmy's childhood and how he perceived his father, in which viewers can deduce Jimmy's philosophy was to avoid becoming sucker himself and ultimately is at the forefront of the struggle between Jimmy and his older brother Chuck and drives the main plots for the two characters for the first three seasons.
From Chuck's viewpoint, Jimmy, also once known in Chicago as, "slippin Jimmy" is a failure and con artist, as Chuck saved Jimmy from jail and in which Jimmy returned the favor by moving to Albuquerque, New Mexico working in Chuck's and Howard's lawfirm (HHM) mailroom in an effort for Jimmy to clean up his life and do things "right" (referring to morality and the law), which Jimmy tries to do, up until a point, as it appears no matter how well-intended Jimmy is, Chuck constantly degrades and nitpicks at Jimmy while he acts as a smug martyr.
The two then have an ongoing rivalry, where Jimmy often seeks Chuck's praise or approval. Upon becoming a lawyer himself, Jimmy's success tends to be the cause of Chuck's electromagnetic allergy disorder, as it turns out to be psychosomatic.
It is then true that viewers often see Jimmy do things like this, when he feels guilty or insecure. He tends to be a likeable self-deprecating masochist, and instead of taking responsibility for his mishaps and failures, he instead tends to set other people up to make them look, the way he feels and sometimes this also works as "self sabotage". It's a way for him to absolve himself from the guilt. The situation is no different here, except that is deeply personal and because of the level at which he chooses to do this.
So back to your real question,
why he was so berating to the owners?
He is doing this to the degree he does, because of the events that transpired in the season 3 finale and possibly because of what Howard told him in the previous episode.
In the season three finale, Chuck tells Jimmy that Jimmy never
mattered all that much to him and then succumbs to his psychosis by
completely dismantling his house and ultimately shaking a lantern off
a table that caused the whole place to go up in flames with Chuck
But to further this, one reason Chuck gets so upset is because Jimmy
screwed over Chuck's insurance policy, which caused Howard to force
Chuck to retire from their law firm. Howard also believes that Chuck
commited suicide and intentionally sought to kill himself. He tells
Jimmy shortly after Jimmy finds out about Chuck's death.
Further Unpacking the Scene:
Some proof of this may be evident by Jimmy's career field choice in this scene, as he decides to go for an interview at a Copy Machine Sales Company of all places, which is ironic, because of the events from the end of season 2; where Chuck, despite his electromagnetic disorder, goes to a 24 hour copying place (like Kinko's) that he believes Jimmy had previously been, trying to prove that Jimmy changed an address to help Kim secure "Mesa Verde" (Bank Client). Chuck gets extremely dizzy when confronting the employee and hits his head hard before collapsing onto the floor!
One also has to consider that Jimmy used to go through this very cycle with Chuck and now Chuck is not here. So to deal with this loss, which he is partially responsible for, Jimmy went to people whom he can symbolically identify himself and Chuck with simultaneously and reenacted the cycle with them.
Another reason that this is happening this way also has to do with where we are in the story. Season 4 is much closer and is overlapping with the Breaking Bad universe, and therefore the events in the season 3 finale give a bigger shift for the transition of Jimmy turning into Saul Goodman to occur. His personality in the second part of this scene is much closer to that persona in terms of becoming more manic.