> someone coming from a different culture, a culture that is less refined than one's own culture, and so the migrant would make everything in one's homeland worse

Why should one have to think a different culture and people are inferior, to want to preserve one's own culture and people? If it was environmentalists worried about native wolves being slowly displaced by an invasive breed of dingo, would you call them wolf-supremacists? Certainly nobody accused Kashmir of supremacy when they worried about immigration [1].

Or are human groups so unique, the first of their kind in the entire animal kingdom, that they actually benefit from a competing group moving into their territory?

[1] Kashmir’s new status could bring demographic change, drawing comparisons to the West Bank - https://www.washingtonpost.com/world/2019/08/08/kashmirs-new...