Posting content that was AI-generated fulfills both requirements of what plagiarism is: You took the work from someone else, and you didn't credit the original source. That's plagiarism.
If at least the majority of the members of this forum thought so, then there would be no problem with such posts - but from the posts above you can read very strange opinions, and it is not an isolated opinion - some members of the forum say that they only make their posts
"more pleasing to the eye" " with the help of AI and that they see nothing wrong with that. Those cultural differences that you wrote about regarding plagiarism can definitely be applied to the use of AI tools.
The only issue here is that AI-generated plagiarism might be difficult to prove. You are again relying on automated tools to check and tell you whether a piece of text is or isn't created by a different automated tool. Do you understand what I am saying? It's not reliable, and can't be controlled and verified by a human. But if there was a database of everything an AI site ever generated, it would make things much easier in terms of proving plagiarism.
It's true, there is currently no completely reliable way that can be used to detect content created with AI, but there is a set of tools that can be used and whose results can then be compared. I also recently accused someone of using AI, and it turned out that even two detectors showed bad detection - which means that we should be very careful and try to check with as many detectors as possible.
In my opinion, the bigger problem is that the forum's administration has not made a clear decision about this, and that practically means that anyone who reads the unofficial rules of the forum thinks that content created with AI is allowed.