The sites most vulnerable to spam are ones that a) have a critical mass of readership, especially dumb readership that will click on ridiculous spam links, and b) ones not run by people who are active contributors to the field of spam filtering.
i cant see how not being an active contributor to the field of spam filtering make your site vulnerable to spammers. Not be vigilant against spammers yes, but you don't need to be in the industry to combat this problem. the dumb readership comment speaks for itself, lets get off the high horse bro.
I think the point was merely that pg has spent a lot of time thinking about the problem of spam (it's one of the things he's famous for), he also wrote the software that runs HN, and when those two facts combine you end up with software that has many mechanisms for automatically preventing spam. It's just that being involved in the fight against spam means his site probably makes use of more cutting edge techniques than sites built by folks who have never dealt with spam before. HN probably also has a much higher "editor to submitter" ratio than most sites, and so a human that has privileges needed to kill spam usually sees it long before it hits the front page.
