If you are not able to prove ownership of a bitcoin address .... you're not satoshi 
Proving ownership of an address proves... ownership of an address. Nothing more. Whoever signs Satoshi's address could have obtained the private keys from him.
Ultimately the only power Satoshi has left is his however many coins so it would be relevant. Beyond that he's been left far behind and that's what he wanted all along.
I'm not saying I believe Craig but isn't it possible that the keys were lost or destroyed. There is a lot of speculation that his partner Dave had them until he died as well.