Maybe more principally, make up your mind what kind of trading you want to do: trading for (moderate) USD gains, and securing them as soon as possible? Then selling now probably wasn't a terrible choice actually.
He bought those coins at $660, so he cemented a net loss. I think it's always a terrible choice to start trading again when you know and admit to yourself you are completely crap at it. It just makes no sense to me at all. Considering Mat wasn't even able to hold his coins in cold storage for a week I doubt he will be able to when he gets lucky to get in lower. Something will happen to shake him out of his position again, you can pretty much bet on it.
I just meant that, right this moment, I see a slightly higher chance for a price drop than for a price increase, though I believe the upside potential to be a lot higher than the downside potential, so I'm personally not tempted to sell. So in that sense, if he'd be disciplined enough to trade like that, it might make sense to be in fiat now to make a quick buck during a drop. But agreed on the rest. Mat kind of seems to have some decent intuition for the market. He just completely lacks trading discipline and emotional control, from what I can tell.