With limit down (or up) moves, the market would never give you a chance to bail out at the price you wanted.
And next thing you know, you have margin calls coming on all sides.
In the past, I think he said he's in a trade for a short period of time; I think 10 minutes or thereabouts. So odds of a limit move happening while he's in a trade are pretty slim. And if the market is that volatile, he probably wouldn't be trading anyway.