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.
This bots arenot inventing that from thin air, they are using copy-pasting and combining what other human being wrote before in different websites.
Basically all AI bots are doing the same thing as a human who is doing plagiarism but than makes little changes and mistakes to make it more human-like and harder to detect for plagiarism.