Oh, sure, I mean, we can play Monopoly as Risk if we all sit down and agree to an extensive set of house rules before play. That we can agree to play whatever doesn't mean that what we end up playing is the thing we started from. I can make up whatever I want, and say that you cannot kill orcs in 5e, no matter what, and it's exactly as correct as your conjecture here -- it's not 5e at all, or the system, but rather something else we've tacked onto it.
For the base system, it is absolutely the GM's authority to alter the premise after the use of a Plot Point (provided that optional rule is in place). This may be a dick move, but it's exactly aligned to the authorities of the GM and they are not abusing the system nor breaking any rules if they do this.
In PbtA games, or FitD games, though, the GM is actively breaking the rules of the system if they do this. It's not just a dick move, it's violating the game system.
I do, however, love how so many of the critiques of 5e (and the above isn't actually a strong critique, there's a lot of good in this function as well) are met with "but I have a house rule."