True, but there's probably many ways to do this and unless AI content starts falsifying tons of its metadata (which I'm sure would have other consequences), there's definitely a way.

Plus other sites that link to the content could also give away it's date of creation, which is out of the control of the AI content.

I have heard of a forum (I believe it was Physics Forums) which was very popular in the older days of the internet where some of the older posts were actually edited so that they were completely rewritten with new content. I forget what the reasoning behind it was, but it did feel shady and unethical. If I remember correctly, the impetus behind it was that the website probably went under new ownership and the new owners felt that it was okay to take over the accounts of people who hadn't logged on in several years and to completely rewrite the content of their posts.

I believe I learned about it through HN, and it was this blog post: https://hallofdreams.org/posts/physicsforums/

It kind of reminds me of why some people really covet older accounts when they are trying to do a social engineering attack.