I'm not familiar with all the details, but don't you just have an absence of proof that it's X?
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.