I'd argue chatbot verbal diarrhea is plagiarism by definition. I've already seen claims against those companies for using data as input without permission (which is a copyright issue). They reproduce bits and pieces of other data without sharing the original source (which is a plagiarism issue).
It is even worse than usual human plagiarism since it is more likely using copy-paste from multiple websites and than it combines everything into one single post without providing any source links.
I wouldn't be surprised if developers made AI to replace some words to make it look different, and same tactics was used by human plagiators for years.
Well, both plagiarism and AI posts should be prohibited in the forum. Otherwise, everyone will get used to it and may think it’s acceptable inside and outside the forum.