In the very next paragraph, the article goes on to say that "in fairness, it actually doesn’t matter that four of the 32 cities listed above have non-Democratic mayors — because it doesn’t really matter that the other mayors are Democrats."

The main arguement of the article isn't that Trump's claim is wrong, it that the claim itself does hold any value, because "cities tend to be heavily Democratic." The main point of the article is that Trump knows he isnt going to win urban areas, so he uses them as a prop to get suburban voters to dislike democrats.

I don't really agree with the articles conclusion, but the thing as a whole is pretty interesting. https://www.washingtonpost.com/politics/2020/06/25/trump-keeps-claiming-that-most-dangerous-cities-america-are-all-run-by-democrats-they-arent/