That's not accurate. BF is great but it's not a parallel HN where a secret world of similar discussion is going on. To overgeneralize a bit, topics there tend to be narrower and oriented around specific concrete situations. A lot of it is about hiring, offices, things like that. There are a lot of Launch posts, especially by startups in the current batch who don't feel ready to go to a bigger format yet.

Actually I think YC would probably like BF to become more HN-like and may find ways to encourage intellectually richer discussion (I've been asked a few times to advise about that). But if YC founders aren't posting to HN, it's not because they're busy participating in an alternative-universe HN. It's more likely as the GP said, that they're busy working—as they should be.