It’s explained right there in the PR:

> The disclosure is to help maintainers assess how much attention to give a PR. While we aren't obligated to in any way, I try to assist inexperienced contributors and coach them to the finish line, because getting a PR accepted is an achievement to be proud of. But if it's just an AI on the other side, I don't need to put in this effort, and it's rude to trick me into doing so.

That is not true of books, search engines, stack overflow, or talking to a worker, because in all those cases you still had to do the work yourself of comprehending, preparing, and submitting the patch. This is also why they ask for a disclosure of “the extent to which AI assistance was used”. What about that isn’t clear to you?