I'd think that they mixed and sold a portion initially and are selling them slowly, as to avoid any kind of tracking and to not garner much attention. It's definitely possible that they've already sold their coins, but then again, nobody will truly know. It might even be the thing that's keeping the price of bitcoin down, even though I admit even that sounds quite ridiculous.
Im quite sure on myself that they already sold part or all of those coins using mixers because as we observe after on bitfinex hack the price drops drastically means those coins are being sold on the current price. If those hackers would tend to hold those coins price would not drop drastically. IMHO