>They must dump the coins on the market right away to less volatility risk.
If they had any reasonable volume wouldn't it make more sense to stagger their selling? On a long enough timeline it'd probably even out, especially if they did something more clever than "sell X% every Y minutes"
They may have agreements with private buyers like the SecondMarket Bitcoin trust that would help move a lot of coins w/o moving the market and being exposed to second by second price changes.
Did they check the exchange rate for buying on Coinbase or selling. Coinbase might be playing with the exchange rate too.