I don't think it's as explicitly simple as the Progressives make it out to be. I think it's not really even a relevant issue so long as a government represents it's citizenry. Worse the Left applies this selectively and choses which groups should be doing this, which shouldn't, and what groups are real groups, and what groups aren't.

It doesn't make sense to me to abolish the UK because Wales isn't a separate country; or worse: that Northern Ireland isn't a separate country. If we run by the actual concept of what the Progressives are saying, the UK should be broken up into dozens of different states. The US should probably be over a thousand different countries.

This is, of course, nonsense. In reality, the Left picks and chooses arbitrarily. This is basically what happened with Germany in the Interwar period. Technically, Germans are made up of dozens of different ethnicities. But, we declare that "German" is the real ethnic group, not Bravarian. Also, Austrian isn't real, and is actually German. But Danish is a separate group from Germans, and the Danes are oppressing Germany, so we can invade.

The whole "self-determination" concept as the Progressives apply it, is just another rhetorical device to conquer, just like everything else they do.