It means that they looked at the price of Bitcoin and saw that, if they had sold at that price, they would have lost $51mm.

This does not mean that they sold the Bitcoins, but that they had to set aside a reserve of money for a potential future loss on their Bitcoins.

This has nothing to do with their propensity to sell their Bitcoins or their expectations on the price of Bitcoins. Instead, it has to do with the way accounting principles work and the need to mark-to-market financial investments.