I don't understand. What's the troll here? Are you offended? Why?

The typical demonym is inaccurate.

It is more or less “USian” in some other languages, and no one raises an eyebrow. 美国人, 米国人, etc.

I'm of that opinion too. The framing behind the gag is wrong anyway. Americans call ourselves "Americans" because we were the American colonies of Great Britain. That is: "Are you English? No, I'm American". It's an adjective, and a properly constructed one. And when they named the country it was the United States OF America, states in America that were united in a single government.

The attempt to gin up outrage that somehow there was ever an attempt to appropriate the name of the whole continent "for ourselves" is just fake. That's not ever what the terms meant.

There's plenty to complain about regarding American ego and hubris, but this isn't right.

Basically you're 100% right. This is trolling, and people should stop.