Do progressive really believe there is no situation in which secession is morally supportable? If not, how do you square this with support for separatist movements or self-determination claims in other nations?
Well, I'd argue that supporting those is deeply overrated most of the time and that self-determination is not inherently progressive.
Check your sig and username brah
*Inherently.* The issue at hand is human rights, democracy, and good government. If those are satisfied, it is hard to justify secession particularly when it might put those values at risk. The ROC, after all, would prefer to be ruling Mainland China using those values. If you think progressive/liberal values ought to be universalisable, you'll inevitable run into conflicts with self-determinists.
But does the US at present have human rights, democracy, and good government? With all respect to the Biden partisans, I think we talk a good game, but we don't really have it.
