Making a hash of the release is just a small part of it (and is the first part of what they are doing).
The trick is to be confident that you're getting the same hash as everyone else - and that's what requiring a proof that it be added to a CT logs gives you some level of assurance about.
The trick is to be confident that you're getting the same hash as everyone else - and that's what requiring a proof that it be added to a CT logs gives you some level of assurance about.