Presume all you've said is true, which, for various factious or groups of Uyghurs likely was true.

That still doesn't establish it as the distinguishing factor as to why the US declared them as terrorist. I fought in the YPG in the Syrian Civil War, an ally of the USA. Guess what, there were those who looked 13,14,15 usually because the Syrian government or ISIS had incapacitated their parents somehow. Mostly they were way back as token guards at training outposts but I also saw some near the front. The YPG also had to ally with a bunch of nasty theocratic arab militias to survive, in fact, that's why the SDF/YPG just got largely wiped out because the consolidation of the rebels in Damascus resulted in their arab allies turning their back. (In fact, Wikipedia page says IS is opponent of Uyghur militia). And I won't even get into the fact that the YPG and PKK are ideologically and pragmatically incredibly similar, yet PKK is magically a terrorist and the YPG is a brave US ally, one gets the blame anytime a Kurdish person does something horrible against innocent people and one doesn't.

As sister poster alludes, the US has never had an issue allying with "terrorists" when it suits their goals. Especially when fighting against USSR.

So knowing that this isn't the distinguishing factor, can you point to any other present-day armed group in or of China that has credible potential for an armed political uprising that hasn't been declared a terrorist? There might be one, I just don't know who they are, but I am very interested to read about them.

To me it looks like the difference is that they were a credible threat of violence against China, not that they have slaughtered innocent people which the USA and China has done as have many of US allies.