SubredditSimulator was based on older generative algorithms, so everyone could fairly easily tell that it was rubbish. When SubredditSimulator got shut down, someone made a new one based on GPT-2 (I think) and everyone was like "OK, this is getting harder to distinguish from real people".
I'm betting someone has made even more advanced bots by now. I'm betting someone's also not concerned about telling other users upfront that they're bots, and they're not confining them into specific subs. Now, the only reason I'm not accusing Reddit Inc themselves of building these bots is that they aren't exactly a bastion of software engineering excellence; the site barely works as is.