What does it matter what some historical US nationalist movement was going for? "Ethnic nationalism" is clearly what's gaining traction the world over, and being discussed. They use the not-unfamiliar conflation of race/religion/nationality based on some ethnic mythos, to equate patriotism with purity of a local monoculture. Is there some "true nationalist" group out there of any prominence?
It's "this state should be run for the benefit of <ethnic group>, and anyone else should be purged". It's about ethnic groups, not citizenship.