I think the point they were making is that you keep double spending until it does. You profit or you profit very very big. It doesn't seem like news of a double spend would ever cause a pop.
I would expect the probability of a crash to go down with each attack. The first attack is novel but the nth is not.
But how do you go about shorting these coins? There's not an obvious way to short them, like traditional stocks.
Cryptocurrency exchanges are pretty involved nowadays, iirc they offer short on most