I was going to say something about wanting to avoid echo chambers and then... I read the post history from the admin over there. sighs
I think it's appropriate to defederate. Doing so doesn't stop their admin, or anyone else, from creating an account on any other instance. Play by the rules of civility in the fediverse and no worries. If they can't, then it will be dealt with via bans.
If they want a maga island to themselves, then so be it.