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)
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.