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.
According to the Oxford Dictionary, "Plagiarism is the practice of taking someone else's work or ideas and passing them off as one's own"
From this definition, we can say that posting an AI-generated post is plagiarism because they took a work that's not theirs and posted it as theirs. It's no different from taking content on the internet and posting it as mine. Someone or something else wrote that, so if you must use it, you have to reference where it came from.
So I think the forum is right to classify AI-generated posts that are without referring as plagiarism and award the same punishment to all parties.
To me, it's even easier to spot an AI-generated post. I've reported more AI-generated posts than I have reported a plagiarised post from another website or article. Once those kinds of posts as reported and they are confirmed, they're deleted immediately.