Actually, because of network effects HN could quite well have better discussions than other sites despite having a moderation policy that's worse than them, so the quality of discussion here isn't evidence they're doing it right. Since most of the users who foster good discussions are on HN, the discussions here will inevitably be better than competing sites, meaning new users will join this site and not other sites. There's a kind of self-reinforcing cycle where everyone often ends up on sites that are, frankly, awful in one or more ways.

(For example, remember Groklaw? That was one of them - the site owner shadowbanned anyone who made good arguments and presented strong new evidence, both when they contradicted her and when they agreed with her, in order to make herself stand out more when she did the same. She also shadowbanned anyone who remarked on these disappearances, so most regulars had no way of realising this was happening. As far as ordinary end users could tell she was just so much better than anyone else that she was indispensable to the fight against SCO. Since all detailed discussion of the SCO lawsuit was on her site, nearly everyone who wanted to discuss the details did so there too and there was no way anyone could establish an alternative.)