The difference is that the U.S. left reflexively supports "self-determination" for Timor, but when the words 'independence for Tibet' scrolls over the screen it causes everyone's computers to instantly reboot.
If it isn't obvious by now, the application of 'self-determination' is inconsistent, since nobody can explain why it is right for Timor and not Tibet. Which means that 'self- determination' is not understood as a good thing in itself but as a means to some different end. So there is really NO real support for self-determination per se. Sort of like Prussian support for Lenin.
Guess you have to be a left-con to faithfully uphold the people's right to nationhood.
On the other hand, you could follow Mat F and others "criticism" of universalism and insist that when I uphold class I am really stumping for 'Eurocentrism' and/or white nationalism. Or when I say black I really mean white. By contrast, when they say nationalism, I think that's what they really do mean. Rather than universalism or class.