He must have paid above market rate at the time of the auction and below the current market rate (ie, between $570 and $650). If he discloses his below current market rate bid, bitcoin price will fall towards his bid level. His new fortune will shrink by millions in fiat terms.
This has to be it.
Actually, that is complete nonsense. We know from failed bids that he paid more than $675. Even if he had got the coins below market rate, he could not sell them without crashing the market and losing money overall. And this isn't his "new fortune." He already had a fortune, he just exchanged it for bitcoin.
Finally, he's already publicly discussed his plans for the coins. He is a professional. He didn't get control of millions of dollars formulating the sort of ridiculous conspiracy-laden get-rich-quick schemes that everyone on this forum seems to be obsessed with.