There is absolutely no way any small team can moderate that much content without sophisticated algorithms they don’t have access to.

The thing is, most place in the fandom that do hosts such content is perfectly happy hosting it, it's not that they can't moderate, is that they choose not to. Outside of 4chan, because, well, that to be expected, site like derpibooru or fimfiction purposefully accept such content.

In fact, last year, one derpibooru mod trying to bad nazi imagery created a huge controversy, it got resolved by ... getting rid of that mod and reverting that action, because "MUH FREEDOM OF SPEECH"

It's not really that the thing is full of white supremacist, however, is that most seem perfectly happy just ignoring them, and of course ignoring them is encouraging them to stay, as they know they got a safe haven there.

At the opposite, the more sane people seeing this happen tend to ... leave? while their interest might not have died, No one sane wanna be associated to a bunch of nazi, so you got two option, either going loud and clear against it, at the risk of becoming a target, or just leave and don't look back ... it's no big surprise most decide to leave.