I don't know how this works under the hood but it seems like no matter how it works, it could be gamed quite easily.
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 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.