Who decides what speech threatens the government? The government.

Uhuh. In other words, governments restrict speech that they think threatens them.

I can say fuck Joe Biden, fuck Donald Trump, and fuck every member of Congress and the Supreme Court. Can you point me towards someone living in China who’s comfortable openly saying “fuck Xi Jinping?”

First off, how many Chinese people have you actually talked to? You know there are Chinese people on the internet that you can talk to, right? And foreign exchange students? You can even visit the country if you want.

And yes, you're free to say things that don't actually threaten the US, like saying fuck Trump or Fuck Biden. You're allowed to be as ineffectual as you'd like. Compare your statements to all the black lives matter organizers who've been found to commit suicide by bullets to the back of the head or public hanging from trees.