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.