I have a theory that a large number of old accounts that were abandoned/not used got taken over at some point and are being used for most AI assisted comments. I just don't care enough to audit the comments of all the users. It was more obvious over the spring with all of the political post and how volatile the voting on comments would be plus other patterns that stuck out

Another possibility is they were registered as sleeper accounts in the first place. Most web platforms make it so cheap to sign up that there's no reason why an organized actor wouldn't collect accounts for potential future activation. For every dozen sites that never take off, there might be one that does, and then the actor can start to activate the accounts they stockpiled over the years for the low low price of nothing.

One way to try mitigate this is binding accounts to phone numbers and binding phone numbers to government ID, but while that kind of thing works to some degree in places like China where the audience for a site is primarily domestic, it's harder to do on an international forum. As usual, the signup mechanisms that preserve people's freedoms and (relative) anonymity also leave the system wide open to abuse.

Before I worked in this space I thought the answer would simply be to charge a nominal fee, like a dollar a year, but while that dissuades casual trolls, it is no hurdle at all for organized crime, state actors etc.

That is a good one, and you really may be on to something. I've spotted this as well but didn't connect the dots in the way that you have. One example was an account that was made in 2014 that made one comment that suddenly came alive and started spouting all kinds of bs, others follow similar patterns.