That would prove the message came directly from the LLM output.
That at least would be more difficult to game than a captcha which could be MITM'd.
It doesn’t really matter, though: you can ask a model to rewrite your text in its own words.
So either moltbook requires that AI agents upload themselves to it to be executed in a sandbox, or else we have a test that can be repurposed to answer whether God exists.
https://www.smbc-comics.com/comic/2013-06-05
https://www.smbc-comics.com/comic/captcha
which may be either funner or scarier in light of the actual existence of Moltbook.