Long-story short is that it's not always obvious how the market will react to releases. Some of the hackers only traded with a ~70% win-rate after holding the releases.
70% is still very very high. They didnt make billions because they probably started with little capital.
70% is extremely high, you can easily get rich with a 51% advantage and making enough small bets to invoke the law of large numbers.
That, and depending on the companies they got data on (i.e. any number of widely traded stocks with large market caps), they could have had a LOT of volume to play with. This is a big deal and it's mind-boggling that the breach happened.
I'm guessing they didn't have a lot of money at their disposal.