I am a honestly a bit surprised by your takeaway from my story. I don't say a single word about my perception of racism in North Dakota in this piece.
What I am mentioning is the general American discourse on race and society, because I am addressing an American audience. Contemporary American debate tends to frame things in racial terms very quickly.
My stay in North Dakota didn't really shape my views on American perspectives on race. For instance my anecdote about the trashcan placement in the kitchen was from a visit to San Fransico to North Dakota. The conversation was between a white and a black girl, and I didn't perceive them as racist towards each other.
I cannot say I find Andreas Hofer perspective on racism very convincing. If you look at world istory it seems clear that racism is far more affected by much more modern and recent events. Scientific racism for instance developed strongly after Darwinianism got established. It was a time of Western absolute dominance in the world and it is not surprising that people sought racial explanations for that. People are always good at looking for excuses for bad things they do. Slavery needed an excuse and thus some busy bees found ways of abusing Darwinianism to that end.
As multicultural societies have evolved in Western democracies we have also seen racism decline as there was been deliberate attempts to counter it. Racism may be more visible in farming societies because they tend to develop stronger hiearchies an be more warlike. Jared Diamond writes extensively about this in Guns, Germs and Steel.