HN is boring enough ( Java API lawsuits / Firefox perf results ) that large groups of people will never read it. So it doesn't have to defend against as big of an assault of bad actors as Reddit/FB/Twitter do. So they can curate the community with actions that scale up just a little. Kind of like a Reddit mod with more resources and power.
Possible to have small bad communities no? I'd argue % of bad actors (vs size) is the tipping metric. Also smaller communities like HN probably have a larger % of bad actors (small denominator).