"Slept with Lumbergh" makes no sense

I always felt there's something a bit off about the car scene, where the protagonist is angry about Jennifer's character having slept with Lumbergh.

First, why doesn't J's character know it's not the same Lumbergh? Wouldn't she know the Lumbergh she slept with doesn't work at Initech, let alone be her current boyfriend's boss?

Second, wouldn't (I will just call her 'J' for short) J know what or who her boyfriend's boss is anyway, or at least know what he's like, based on all the descriptions and complaints (this guy is so whiny, there's NO WAY he hasn't complained about Lumbergh to his current girlfriend)?

Third, why is her reaction something like 'that's none of your business!', instead of 'What? Lumbergh works at YOUR company and is your boss? Wow, I should come with you to work so I can meet him, haven't seen him for a long time' or something?

Wouldn't she be nostalgic about Lumbergh and take this opportunity to want to see how he's doing now? I mean, she's already SHOWN to be a bit nostalgic about him, the way he says 'Ooh, right, Lumbergh!' (or something along those lines, can't remember exactly)

Wouldn't she at least QUESTION the boyfriend about Lumbergh, what he looks like now, how he's doing, and all that?

Being his BOSS, she might even have a rekindled flame burning... I mean, even the actual Lumbergh drives a Porsche, so why would this waitress choose a man, whose ambition is to 'do nothing' and 'not pay bills', which will surely land him homeless or something, when he could be with someone that drives an expensive luxury sports car?

In any case, this 'arguing scene' seems completely unrealistic and inorganic, the things both say, and the way they say them, and so on. Sure, she would get angry and feel insulted and accused, but there's no way the MISUNDERSTANDING of which Lumbergh we're talking about, would just go UNRESOLVED in real life.

The only reason it does, is.. 'so that the movie can happen' (how many movies make no sense because of this rule? Sigh.)

Realistically, Lumbergh's first name would definitely come up at some point, and there's no way J would be THIS clueless about her ex, women usually keep track of what their exes are doing, and message them from time to time, even if the exes were 'losers' at the time, because things change and women have to know everything..

In any case, this scene just doesn't make any sense.