Szabo reposted this blog post [1] on December 27, 2008
The Bitcoin genesis block was created Jan 4, 2009 [2]
At that time there was about say 3 people in the world interested in these ideas. Make your judgement and consider the probabilities for P(Szabo = Satoshi) and P(Szabo != Satoshi).
I think if you read up on all the articles and convince yourself. In [3] Szabo said there is a lot similarities with Bitcoin and Bitgold, and he talked about the differences in interviews.