I don't think that's quite right. In Lewis's thesis, you only really encounter the in/out question by interacting with people who are in. By your definition, interacting on HN puts you in already. And "and have participated semi-regularly in discussions without getting banned" is a much lower bar than anything Lewis is talking about. You'd have a much better point w.r.t. lobste.rs or other invite-only forums, but the whole concept is dubious without private discussions.
Granted, but I'm not saying that just posting and commenting on HN, or passing the various gates, constitutes being 'in'. Even having high karma isn't 'in'. I don't know if I can articulate what counts as being 'in', but certainly participation in a YC round is part of it.
No, that actually kind of is what you were saying in your previous comment, about what "the forum itself enforces".
You could identify a set of people who are widely respected and listened to, but that's a different thing again from what Lewis is talking about. YC itself is arguably closer, but only tenuously connected in practice to HN social dynamics. I certainly couldn't tell you which members I pay attention to have been through YC (except for the ones who definitely haven't).
