They hold a lot of US debt, as opposed to USD currency (I'm not sure what damage they can do due to the latter). But actually Japan holds more US debt than China, and the UK has almost as much.
Any country could start a massive sell-off of its US debt, but then they would be cutting off their nose to spite their face, unless there was some equivalent geopolitical benefit. They would probably be selling at a substantial loss and/or reducing the value of the US debt that they still held.
It's a bit like Elon Musk's wealth (which, for the avoidance of doubt, I think is obscene). He can't convert the $500 billion or whatever his companies are worth into cash and buy Freddos with it, because a large part of the value of the shares is the very fact that he is prepared to hold on to them.
Ditto, to a lesser extent, with using Norway's sovereign wealth fund to finance Ukraine and European defence more widely (which has been proposed on the basis that if this isn't the "rainy day" that you're saving for, what is?). It's worth $1.8 trillion, but if you try to unload all of that at once, you're not going to get $1.8 trillion for it, partly because of market confidence and partly because there just isn't that amount of cash sloshing around waiting to buy blue chip stocks.