Bitcoin has a much higher tolerance for dumps and large sell offs due to its higher demand and higher transactional volume. The Winklevoss Twins could sell off all their BTC and it might not make much of a dent depending on the quantity and price of buy walls on exchanges. There are many other big holders of BTC such as bulgaria's government which held $3-$4 billion in bitcoin it confiscated from criminals.
I think the USA once held large amounts of bitcoin seized from silk road and other illicit enterprises. Last I heard, those funds were sold off years ago when bitcoin was worth much less.
With coinbase reported to be receiving around 100,000 new sign ups per day there could well be enough demand for bitcoin to survive large dumps without sustaining much loss in value.
True, it's more likely they'll just hold on to those coz there's really not much benefit to selling them all out in one go.
Sucks for the US government. Well, no one really expected it to be this big anyway. Plenty of people got scared it'll collapse and started selling even when it was below $100.