Maybe he thought no one would actually look back and see all the transactions with MtGox wallets to trace the coins they had, I don't know. But the openness of the Blockchain proves that he has access to at least ~90k BTC in wallets which are proven to be MtGox wallets. Such as this 50k here: https://blockchain.info/address/1P3S1grZYmcqYDuaEDVDYobJ5Fx85E9fE9

The full details of how people are tracing it are in this article: http://www.hackingdaily.com/2014/02/mtgox-speculations.html. A lot of it goes back to when he was proving he had the coins in 2011 by doing this transaction: https://blockchain.info/tx/3a1b9e330d32fef1ee42f8e86420d2be978bbe0dc5862f17da9027cf9e11f8c4 and you can follow the coins from there and spot them moving into wallets which top off the known hotwallets from MtGox, which also makes it easy to spot the "Cold Storage" wallets.

There is still more which can probably be found as well by going through known MtGox wallets and following the transactions. Every word out of his mouth ever since he claimed to only have access to a "small" amount is just lies. Unless they lost the private keys to these wallets, which I find extremely unlikely.