It's one of two ways. It's either the hash algorithm hashes are strong enough to prevent manufactured hash collisions (not just accidental ones) and the gov could convict on the hashes alone, meaning the files aren't needed and the All Writs Act is not appropriate, so he's not in contempt. OR the hashes wouldn't stand the trial by themselves, and leaves a much weaker argument for the need to decrypt because you're less certain that it would contain the files you think it does.

I think the specifics in this case, Like the sister's testimony, go against the second possibly weakening their argument.