Rudeus is just a random lad, who even somehow avoided Orsted's godly divination power. How would Rudeus know the Man-God exist if not by being in contact with him? If a librarian knows, it's likely due to the books and the flow of information in the trade. Of course a librarian could also be in direct contact, but that's only the logical secondary option that would need to be investigated. That being said, it's possible Orsted would kill the librarian in any case, so that the person wouldn't spread the knowledge. However, in Rudeus's case, there's no quick alternative option to explain it.