Explicitly safe spaces in real life typically aren’t open to having strangers walk in off the street, even if they have a bouncer to throw problematic people out.
and this is the issue with beehaw isn't it, i'm not judging them wanting to carve out a vetted/secure and controlled space, it's entirely understandable given their audience and purpose, but then why are you building such a community on federated software? that sounds impossible! part of the point of federation is to allow cross-talk from other spaces and communities. if you're going to defederate and wall off from 400 instances (as OP claims) then beehaw should transition to vanilla forum software, this would save themselves all the headache.
all the best to them ❤️