I think this is best answered by a moderator, but it probably boils down to:
1. Self selection bias. HN isn't a huge outlet, and doesn't attract quite as many spammers as other platforms.
2. Quality moderation. Just flip on the `showdead` flag on your account and scroll through the hundreds of [dead] threads.
3. Some level of auto policing. I know that HN validates some level of page history when counting upvotes.
(i.e. if you navigate directly to a page to upvote, that vote isn't counted)
It does get a lot of spammers. Probably hackers trying to have fun getting around HN restrictions, as well as some traveling spambots. I wouldn't say it's "not huge" - HN regularly pops up at the top of Google searches.
Downvotes do help a lot. dang often refers to them as " white blood cells".
Some new accounts do get flagged as spam accidentally, especially when they sign up to promote themselves in a less spammy manner. Vouches do help here and if it's really bad, and you can email HN if you think someone is wrongly flagged as a spammer.
Incredibly good paid mods, aggressive spam algorithms, even more aggressive downvoting, and a lot of (justified IMHO) gatekeeping. Nothing you couldn’t do yourself for a million or so to start and at least half a million a year to run, assuming you hire brilliantly ;)
1. Self selection bias. HN isn't a huge outlet, and doesn't attract quite as many spammers as other platforms.
2. Quality moderation. Just flip on the `showdead` flag on your account and scroll through the hundreds of [dead] threads.
3. Some level of auto policing. I know that HN validates some level of page history when counting upvotes. (i.e. if you navigate directly to a page to upvote, that vote isn't counted)