Just because the concept is meaningless in the real world doesn't mean it's not a concept who's definition is widely understood.

Should I have put "white person" in quotes? Should I had phrased it like "what would commonly be called a "white person"".

I felt that would be overly pedantic of me and besides you are missing the point: "white people" do exist, it's just wrong to assign any shared attributes to them other than "looks like a white person". And by the way, I don't mean "wrong" in any kind of moral sense, I mean wrong in terms of "factually incorrect".

If he had talked about ghosts instead of "white people" would I have sounded awfully silly if I had said "ghosts don't exist. Ghosts are frequently just photography artifacts or sleep paralysis hallucinations." because I used the word "ghosts" in the next sentence?

So you think that the pre-scientific idea of categorizing people by skin colour and nose and eye features is valid and that the scientific understanding from genetics is "nonsense" that I "want to believe"?

That's both ridiculously presumptuous and ... well, stupid.