Another thing is that Gavin is a trusted individual the Bitcoin community, and CW is someone who back dated PGP keys in Satoshi's name. A zero-knowledge proof would perhaps have been logical with a more anonymous member of the community. But Gavin's word is the only evidence to this whole story.

If Gavin had leaked the signature, that would probably have spawned more speculation that Gavin is Satoshi rather than that CW is.

And presumably the reason to control the signature release is to make a big public bang. Kept secret it is useless. The only reason for that would be to make a public fool out of Gavin. Not very nice any way you look at it.