Well, they didn’t actually witness the author signing anything. They witnessed the numbered piece of paper on a table in a bookstore, and the author may or may not have been present.

To know if this piece of paper was actually touched by the author, we still need verification outside of the blockchain.

It’s not obvious to me that all this has any value. “The author probably blessed a number” certainly doesn’t have the emotional connection of an actual physical signature, for example.

In a way NFTs seem to resemble the medieval church practice of selling indulgences. If you believe in the authority of those mass-produced blessed pieces of paper, they have value.