Earlier quoted context omitted.
I'd suggest: new accounts are read-only for at least a week. Then they can comment (rate limited at first, gradually relaxed) and vote, and then after some additional amount of time and/or karma they can submit a post. Maybe some of these mechanisms are already in place? Bots can probably game this too but drive-by bots maybe won't be patient enough.
It seems easy enough to circumvent: "We're launching our product in 2 weeks, so let the AI create and 'warm up' 20 new HN users so they're ready to shill". It's really not a problem that can be solved easily :(
With a couple few layers of defense, you'll weed out almost all of the bad actors. Without strong monetary incentives for spamming, you also avoid most persistent actors.