This "researcher" doesn't seem to know what end-to-end encryption is, or why what the manufacturer says is true. Their blog says that "[t]he term is generally used for applications that allow some kind of communication between users", but that's not true. The most common type of end-to-end encryption is HTTPS, typically between the user and a web server.
Also, they offer an AI powered service to analyse your output, and state that they use the data for further training. That is well within both expectations of what an AI powered service will be doing, and what their privacy policy says they will do.
I dislike how privacy is treated as a premium product, and how many companies feel entitled to our data, this case is nothing special at all.