Who knows why he left the mined coins from 2009/2010 untouched? It can only be speculation, but remember his motivation -- to create a peer to peer currency. He had to make it look like the new currency was being used, but also not look like an individual couldn't join. He couldn't look like he would hold the majority forever or no one would want to join. He could even have sent those coin off to the void to create the mystery, and drum interest. Perhaps he hid coin behind different riddles and tests to test the resilience of the network. Him mining made it look like there were more users than there were, before the extra nonce tagging him was discovered.