Don't forget he holds > 1.000.000 BTC. It may be hard to spend his coins without revealing some details about his identity to someone. If he is exposed, he doesn't have to care any more and dump, let's say, 100.000 BTC on the market to get some cash.
It is quite possible of course that he has other coins beside those known to be "Satoshi's coins", and has been spending those.