But it demonstrates nothing if no one clicks. The channel may be insecure, compromised and no one knows - not the author or the parties who are supposed to have the secrets.

I see what he's trying to do, but this isn't the way I don't think. Mathematical proofs that verify that a payload is observed, opened, or accessed work. They are deterministic. They are also way more complex. I think this is trying to solve a problem in a simple way but it's still just as nondeterministic as without this solution IMO.