Suppose that claim is not true, or that he was not fastidious in his use of Tor, it seems to me the email service provider would have logs on where his emails were originating from. From his original paper his email is satoshin@gmx.com
I'm not sure who owns gmx.com, but I think it likely that if a sovereign state went after them, they would probably divulge the information. Not that I want this to happen, of course. I hope he's never uncovered and his legend grows with the success of Bitcoin. I hope he did use Tor. I'm just not certain there is evidence for it.
Edit: I am not Satoshi Nakamoto, although I wish I was