The original author is not the poster. Its ChatGPT. If you credit ChatGPT for the post, then its not plagiarism. AI posters are misrepresenting their actual abilities, and most of the time the output is perfunctory blather that doesn't add anything to the conversation (which is why there is a lot of overlap between spam and AI text).
The chatbot AI users never credit their posts to ChatGPT or other AIs because they intend to steal contents created by ChatGPT, AI and make it like their own content.
If they are in a campaign, campaign manager will not pay for posts credited to ChatGPT, AI.
If they are newbies, forum members will not merit these posts if there is credit to ChatGPT, AI.
Intention of ChatGPT, AI users is clear, and they don't forget to add credit but they want to hide source of the content.
So it's plagiarism.