I'm not sure that's true at all, lots of fringe views gain traction on Twitter. I'd imagine more of them still are moderated out of existence.
> So this is the key tension. Outgroups get criticism of themselves every-bloody-where
Yes, and by being a digital town square, Twitter, or any similar platform, would become part of everywhere, with the same consequences for outgroups.
> provide an opening for an outgroup to issue its own civilly-expressed criticism
I agree that outgroups should have this opportunity, but it must be applied to all the outgroups, including those who have a mutual loathing for each other, and those whom we loathe ourselves. Fairness dictates we shouldn't play favourites, in the same vein that the ACLU defended the a Nazi group's right to march through Skokie.
If an outgroup wants a private, safe space to express themselves, that place cannot also purport to be a public square of free expression. It's one or the other.