It is kinda hard to hide something of that scale, in case a prominent member of the Chinese mining community such as Jihan Wu (or anyone else for that matter) is threatened with prison time. Something is going to leak out.
The governments know that, so they won't do anything like co-opting mining farms. They can't even contain some of their territories ... (HK protests, etc.)
why would they care about a leak? i assume the point of such an attack would be to crush confidence in bitcoin. a state-backed attack might shake peoples' confidence all the more.
bitcoin miners in china are fairly concentrated and there's no comparison in numbers to a popular protest movement. large mining farms and pools would be much easier to control.
PoW algo change is one of the last things to change, if it ever gets to that point.
i'd prefer to think we could just "wait out" an attack like this rather than hard forking.