He called me a setter and a nazi out of the blue, and I asked him if he slanders everyone or if he has a reason for saying that about me, then he replies with this legendary quote:
If you’re Amerikan, then you live in the bowels of the single most brutal settler-colonial entity to exist in the modern era. “Settling” is not a once-and-done action; it is a constant state as long as you are on settled land.
As a white American descendant of colonial settlers, they're not entirely wrong, it's just that their anger is misdirected. It should be directed at those in power rather than regular people. It's unreasonable to expect all descendants of colonial settlers to leave, but it's not unreasonable to ask them to be aware of their history and how the systems of power and control that were put in place then are still going strong today.
Given I can't see your original comment they were responding to I'll reserve judgement regarding whether what they said is reasonable.