But "think about the children" has always been one of the tactics used to push this kind of legislation, along with "terrorism" and variants of the two. That doesn't explain why HN is now more willing to believe the ruse.

Because this isn't just a "think of the children" narrative, it's also a "don't you know 4chan has lots of bad people who you personally dislike" narrative.

I would surmise that while 90%+ of guys are porn users, a nearly negligible percent are current or former channers, and most only associate chan sites with fringe groups that took them over.