I wouldn’t even mind so much if all he did was assume he was right. We all have situations where we believe we have the data and have come to a conclusion, and thus argue our point while regarding any counterarguments as flawed.
No, what frustrates me are the one liners with no attempt to actually engage. Very often, his only argument is his single sentence claim. Other than that, it’s just basically implying everyone else is stupid if they disagree.
There are other posters who are very opinionated, especially on topics they really care about. I’m one of them. But we at least lay out our thinking, opening it up so people can point out flaws.
Granted, sometimes it seems to me that we get nitpicks that don’t change the overall argument, or portions of the post are taken out of context and distract from the main point. But the alternative of not stating that reasoning at all isn’t better. It just makes what you say more dismissible.
He also seems easily triggered on any bigotry-related topic. This tactic of one liners seems most common when he’s angry.
Finally, I’ll point out that there is a difference between pointing out that something someone did might be racist, and saying the person themselves was a racist. And there are degrees of racism in a work, from accidental tropes that snuck into your work through the cultural zeitgeist of the time, to purposefully trying to push a racist ideology. People often state the first one, but then the refutations seem to be about the latter.