How do you prevent the "filter bubble" from self-reinforcing the same perspectives?

This is relevant but it's not really. Filter bubbles have always existed, and as you mentioned, clubs etc have always had this.

Modern filter bubble, though, usually refers to an effect which is machine-produced. Algorithms on facebook and youtube actively reinforce beliefs, simply because showing content which a person agrees makes them feel good, and these companies want you to stay feeling good on their platform.

None of that applies to the scuttleverse, so we go back to the status of "PHP forums" and other ancient forms of tribalism. One could argue that filter bubbles have always been bad. That may be true, but SSB is not adding to that problem.

So I'd say the real problem is algorithm-induced for-profit filter bubbles.