I'm not making the second assumption at all, I explicitly disclaimed any opinion on the merits of particular stances on immigration in countries that are not my own:
> I'm not writing this to take a stand on immigration in Europe or anywhere else, but it's important for us in the US to not project our own sensibilities into a completely different context.
As for the first assumption, I'll grant you I was making that. Whether or not the concept of a nation exists in reality, it undeniably exists in people's heads, and it's less obviously artificial in Europe and elsewhere than it is in the United States.
EDIT: Although, note that I didn't actually use the phrase "true nation", I said "true nation state", and I think the existence of a nation state is less up for debate than the existence of a nation.