Invite-only isn't a bad idea, given the audience. This approach may help them keep the overall content quality higher than if they opened it up to everyone.
As for posting it on HN, I think it makes sense for them to try to attract some new viewers--even if they are selective about who they let in. I'd do the same thing.
If you're going to be a gated community then don't post it on HN.
(also, I seriously doubt this ensures "quality".)