Even Craig Wright or Dave Kleiman seem a lot more believable to me than Mark for a Satoshi role. The amount is virtually the same: 648,000 BTC declared missing from MtGox  vs. 650,000 BTC withdrawn from the Seychelles trust by Craig.

As mentioned in the LRB article http://www.lrb.co.uk/v38/n13/andrew-ohagan/the-satoshi-affair, Craig was planning to use these funds to pay for his "research and development stuff." It seems natural that an exchange like MtGox would be chosen to trade them all for fiat. Craig claims to have lost a "large wallet" when the exchange went down.