that's the thing though, he never signed anything. i don't know why you think that now. he faked some signature and refused to show it publicly because he knew if he did do that, people would catch his fraud in matter of seconds. so he paid Gavin to lie for him. it is harder to prove someone lied to prove a cryptographic signature is a lie.
You think Gavin would take some bribe and spread a lie ,what ever Craig showed was convincing enough for him and remember he did have contact with the real Satoshi and if you did have contact with a man as long as he did with Satoshi through any medium you could identify him if you meet him in person that being said i am not convinced either who the real Satoshi is unless i see the proof myself.