I think the idea is to weed out the obvious stuff and have a rule you can point to. When someone opens 20 PRs in a day with an entire novel in the PR description. You can just close them without having to sink effort in to each one. The LLM signs are less in the code itself and the patterns of how the person interacts with the project. A normal contributor might join the IRC, be involved in the project, start on smaller issues and work up. An AI power user just drive by dumps a mountain of content and then leaves.

If a contributor genuinely understands the problem, has tested the code themselves and writes the PR description themselves, it would be indistinguishable from human written code anyway.