On some computers their internal operating system software that creates the random number is not completely random. It produces a very long sequence of numbers, perhaps a list of millions of numbers, but they are in a certain order on the list. To a human looking at one segment of the list the numbers would seem to be totally random. But once you can identify a particular place on the list then it is possible to predict what the next numbers will be. The computer running the slot machine is cycling thru the list very quickly, going from one number to the next on the list. That's why the timing is important when the hackers told the guy to press the spin button. They had reproduced the same list of random numbers the slot machine had created, then they figured out where on the list the machine was at the time (that's what the iphone was doing for them), then they calculated when the next winning spin would occur and were waiting until the machine got to that place on the random number list to tell the guy to press the button. It didn't always work because the man couldn't instantly press the button when they wanted, but it worked often enough.
This is why some things (like software used for encryption or for lottery systems) make the random selection a different way, it is tied to some natural random event. For example, the keno ball machine is a random selection, the balls are jumping around due to air flow and one of them randomly gets selected. No way to predict which ball will get chosen.
These guys did it, the proof is in the money they won, but it takes a computer and special software to be able to do it in a reasonable amount of time. Very clever thing they did.