Can someone explain the "into DnD but also a Zionist" archetype? Why do I keep seeing geeks consuming very-much-not-fascism media, while siding with fascism irl?

Because they don't care. They do a good job at separating the art from the artist, and at times conflating the art with their own views. It's why a lot of bronies are weirdos and fash adjacent. Fascists think when people complaining about their lack of media literacy think every piece of media is anti-capitalism, but that's not true at all. Sometimes a piece of media is simply making fun of these losers with no deeper message and they still refuse to grasp it because they're unable to comprehend that conservatism doesn't create. At least nothing that's widely consumed nowadays.