Maybe he simply lost access to this address ? This is something rather common, sadly. And thus he might have said the truth when he said he sold all his coins.
Why would he? One of the most renown developers of bitcoin has lost access to his wallet? Well, that's bs. And why would we care, though? It's his own decision to sell his coins, and if he kept some, so what? Mike Hearn is a part of bitcoin's history since he's one of the few people who Satoshi exchanged emails with during bitcoin's initial development. Also, bitcoin isn't exclusive to some select people, so I guess it's okay for Mike to have his own share.