This is fundamentally impossible without multiple third parties personally witnessing the act of creation. Consider John Milton was blind and composed Paradise

This is fundamentally impossible without multiple third parties personally witnessing the act of creation. Consider John Milton was blind and composed Paradise Lost by dictating it to another person. Even if you can prove a human, even a specific human, physically performed the act of typing or writing, that doesn't prove they came up with the words themselves. LLMs can whisper in your ear. Though I don't personally believe this, consider the most widely-read book in the history of western civilization still has contested authorship, with many if not most readers convinced God himself dictated the words to the humans who wrote it down. We have no way to prove or disprove that.