That's exactly what makes this cool! Since we know the signed hash is a chunk from the Blockchain that is most assuredly not Sartre, we also know with cryptographic certainty that Wright does not possess a copy of Sartre with that hash. That would be what's known as a preimage attack -- given the hash from the Blockchain, find another SHA256 file with that hash. Wright could not even generate a malicious Sartre document with that property, without having executed a much larger crypto hack than Bitcoin itself.
here[1] is the text that he claims he signed. I wasn't able to tweak it in any reasonable way to make it match the supposed hash. You can give it a go if you want.