The full description says it was originally figured out by a Japanese dude. He merely extrapolated and wrote a guide and made guide videos about it. Reading the description theres a whole lot what can go wrong with it but it's still pretty funky. Essentially the PS2 doesn't truly generate random numbers more it goes through a list of them stored on the system in order so if the list was say:

10
99993
102940935
12203596303021
2
39538
4
2045398

Or so on, when the PS2 requires a random number it merely reads the first number from the list in the example's case 10 and uses that when it gets to the end of a VERY long list it'll go from the start again. There is no real way of telling what these numbers are except that they are likely to be exactly the same in all consoles so as to make coding for the console easier. Once you figure this out and grasp it and know what position/number is needed to manipulate something then it becomes trial, error and simple mathematics/logic to figure out the patterns needed to reproduce this result many times over. This is what he has done. He has obviously figured out how much FFXII relies upon this random number generator and from the start has figured good ways of getting through to everything. You should definitely take a good hour reading through and comprehending the full description to do this otherwise you'll waste a lot of time.