Contrary to an economist's default assumptions, information doesn't travel instantaneously, and not all actors are immediately logical. To put it another way, there's a delay where profitable arbitrage is possible with widely-distributed information because many large players are slow-to-move.

In short, there is reaction time - sometimes a 2-4 day window - between when news is released and share prices get where they're going. So those hackers had even more lead time in executing.