I don't remember the exact quote and I can't find it, but I recall someone describing a similar problem on 4chan itself. There's a thing that happens, in communities that emphasize edgy humor, to newcomers who don't get the jokes and confuse them for genuine beliefs. Those who find them repulsive leave and don't return, while those who find them tolerable (as genuine beliefs) stick around and have those beliefs normalized and reinforced. Over time, the sincere believers come to outnumber the jokers, and what was once a lighthearted satirical community becomes a cesspool of bona fide extremism.
I don't know if it's a fair analogy; as far as I can remember 4chan was always a cesspool of violence with a flimsy veneer of "it's just a joke bro". But I think it would be unwise to assume something like that couldn't happen in a femcel community.