It’s going to be a challenge for them to reach new people to warp to their worldview if they are largely kept away from everyone else.

This only makes sense if you assume that new people would be funneled into extremist Lemmy instances from other Lemmy instances, but it's far more likely that new people will be pointed toward such communities through other paths, and then they won't be exposed to competing ideas.

Don't misunderstand me - I'm not saying that hate speech, racism and sexism should be ignored or allowed to exist alongside other content on Lemmy. I am saying that trying to ignore that, separate ourselves from it, and pretend like it doesn't exist and doesn't affect us, is ultimately counterproductive.