Why not use a bot to automatically post brand new chatgpt-generated fake comments in between edits with fake stuff? Or automate making edits that insert/delete/replace one word with a random word?

There's many different things to do that could trash the data and make it worthless for training AI. With the right tools, this sort of automation can also be made very difficult to detect (randomize the time interval between edits and the time of day on which they occur [normally distributed during prime time]).

That is not how the GDPR works. They have to delete all information upon request or termination of the service.