I agree with that observation when it comes to reddit. But it makes me wonder why HN still contains a high signal to noise ratio despite not having subforums.

I'm just guessing, could it be that it's the top vs new categories? It's difficult to leave the "new" section without interesting and relevant content. If you check the new section, it's full of duplicates, spam, self-promo or otherwise uninteresting submissions with zero comments.

Same goes for comments, bad comments get downvoted and greyed out quickly, whereas great comments get elevated.

The design is a big deterrent to that. HN isn't remotely similar to any modern social media or forums.