It can be treated under plagiarism or even spam.
For me AI generated content in forum fits nicely under rule number 33:
Posting plagiarized content is not allowed.
Yes, but it's not clear plagiarism because the content is not taken from somewhere on the web, or a forum or anywhere, it's just generated through a bot and used as if it's written by the user himself.
You can actually use AI generated text for something and post it in forum, but you should post it with quotes and add note that this text was generated with xyz AI.
That's different and shouldn't be punishable just as how you can provide reference with copied content and don't get punished for plagiarism.