It seems like it? And ngl there's a certain amount of sense to it (or at least people who think about it for one second claim that, whereas people who think about it for three seconds have already moved past it), but it's still a nickname used by outsiders rather than chosen by those to whom it would apply. Plus often used in a denigrating manner such as here - imagine if instead it said:

Good. Cunts must learn that they are not above the law.

Or remember when George Bush went around renaming world leaders whatever he wanted to call them in that moment? Donald Trump is doing that lately too...

Traditionally, names used to have meaning, and USian is not a name that Americans have consented to be called. For me that is the end of the story, but I suppose for others who have more incel-like qualities the argument seems to remain that USians will be called by whatever they feel like and like it (I suppose in gratitude for having been deigned to be given any amount of thought at all?).

the quotes at https://en.wiktionary.org/wiki/USian suggest it has been in use by canadians since 2005. UKians may have picked it up more recently but that term doesn't really work in comparison, unless you want to emphasise that you are specifically including the northern irish