Probably this is already a really basic psychological concept that I re-invented, but anyway.

Sounds like cognitive dissonance to me. I get, I also used to do the ”actually vegan diets aren't as good as you vegoons think” stuff before I figured out that the best way to get rid of cognitive dissonance was to just be vegan.

I never would've gone to spaces specifically for vegans to proclaim how awesome meat was, though. Do these people think there's a Medal of Edgy Contrarianism in it for them or something?