You are moving the goalpost. OC is not talking about thoughts having to be unique. You can very much read someone else's thoughts, examine them critically and come to the conclusion of agreeing with them. You might then write a comment like OC putting these thoughts into your own words – maybe abridging them, placing different emphasize according to what you find most important or even adding new ideas – and quote or paraphrase something you found very fitting.

That is completely different from generating words that are not an expression of your thoughts or at the very best take some concise thoughts from the prompt and stretch them into a lengthy word soup.

Of course there are people who just parrot other people's thoughts without understanding them. That's closer to using an LLM. But you can often see how well someone understands something and how much reasoning of their own they put into it through their writing. And for the writer writing is a very good way to reflect on their own thoughts and to get them into order. Proper writing is thinking.