That is only true for part of the move during the flash crash. I know of several guys if they had gotten out they would have gone instantaneously bankrupt as their positions were deep in the red already. They held on, and guess what. They got no margin calls whatsoever. Their brokers and FCMs should have forced the closure of those positions because they were in violation of exchange mandated requirements. Either the exchanges or the brokers/FCMs let them off the hook which only worked out because the market ticked right back and because of the total carnage and chaos in the market. It would have turned into a huge disaster had the market stayed at the lows or even dropped further. Point of the story is that such behavior put everyone else at great risk.