China only owns a small percentage of the debt. The majority is owned by US individuals and institutions.

If China wanted to sell of its treasuries immediately it could and the world would yawn. It would simply be exchanging an interest bearing bond for non-interest bearing currency... which is stupid, which is why they don't do it.

But the debt is a convenient scary story rich people tell to the poor, so please remember to be afraid instead of wondering why Trump inflated the DoD budget over his term multiple times and nobody blinked, but there's no money for health care during a freaking pandemic.