What do you mean RNG is not random? RNG is not a single thing that people can touch or exists. Or are you saying that random number generation is not actually random?
There are differences between types of random number generators. For example, most computers use algorithms to generate random numbers. If you know the algorithm, you can predict the random number (making it not random). But there are more complicated random number generators out there. For example, the Stars RNG (http://www.pokerstars.com/poker/rng/
) is perhaps one of the best I have ever heard of. The last time I checked, it generates random numbers from 3 sources: 1) A computer algorithm, 2) User inputs (i.e., mouseclicks from millions of stars users), 3) entropy (a truly unpredictable source of data).
It is possible that this casino used a weak RNG and this person knew the algorithm or that this person simply saw the results before they were posted.