My point on the "few thousand insurgents" is more specifically referring to Afghanistan, but I think both apply similarly: that the US has been unable to contain and combat relatively small insurgency movements via direct action, be that boots on the ground or through mechanized warfare. I think if a similar insurgency happened here in America, if say a percent or even less than a percent of Americans engaged in sympathetic action, that it would be nigh impossible to contain given our military's past performance against similar combatants, not even taking into account the nuances of fighting one's own citizenship and the likelihood of disobedience among the state.
Arguments that a scenario could be contained at local, civic levels are valid, but a full-scale conflict between citizens and military would be catastrophic not only for the population but for the administration as well (who wants to blow up all the infrastructure they've built?). The state has a vested interest in not provoking this scenario, which is why I imply the second amendment is important and alive in the US even today although the tides have changed.
In regards to HK, a previous comment above mentioned they have a much more comprehensive "bottom-up" structure of monitoring and enforcing compliance with their citizens. Even if they had a theoretical second amendment, they can still initiate a massive crackdown and have the infrastructure to contain and control their citizenship. Which is frankly terrifying, of course, but the solution isn't necessarily just "give the citizens guns", nor would this ever work on a cultural level.
China does actually have the resources and is in a position to contain civic revolt in nearly all cases, from my point of view. This is vastly different to America, and can't really be easily compared.