I would wager the vast majority are alt accounts of existing users. People who don't want to risk their karma or reputation but who do want to go mask off for certain subjects. After that it's bound to be bots run by HN users. I just don't think HN is so popular that the rush of green accounts popping up actually represents new users. Maybe I'm wrong , though.

Reddit has been shedding its techy enthusiast crowd for the past few years with the combination of policy changes and insufficient moderation against LLM bots. I wonder if that’s contributing.