But how would you get ~real~ friends if not through casual meetups first?

I mean sure, you could use casual meetups to meet new people, but friendship really develops and exists in a 1:1 context. I re-read the root comment and it seems like the person has the right approach initially - meet at a meetup, invite for 1:1 coffee afterward.

But then he invites them to his potluck and I think the friendship dies there. That potluck is like a graveyard of all his potential friendships. It just becomes a "hangout group" i.e. fake friends. Also, every other month is too infrequent.

Personally I think the best way to make friends is at places where you might meet them in a context that gives you sufficient and regular 1:1 time. This is almost always school or work.