For me AI generated content in forum fits nicely under rule number 33:
Posting plagiarized content is not allowed.
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.
Anything else is going to be considered as zero or low value post and cheating.
If detected post gets reported to mods.
Simple.
You have said it all, that AI rules should be under or considered as plagiarism, And many persons I have come across their cases concerning AI has been treating same way as plagiarised post is been treated..I have not seen any value why someone will put in quote of what it extracted with AI and post in the forum, since the post will look as low value post, i don't think it's needful, the only thing is that anyone that's been caught with AI post should face the consequences by the mods [ban he/her acct] and i think that will limit the rate of using artificial intelligent for forum, if many accts is been brought down, people will desist using AI.