If the reputable gambling website cheating the members, and the members know, they will get into the problem, and the member can chase the site with help from other people who know about the law. Yes, we don't know if the system cheats or not, but we can anticipate from the cheat by not using too big money for playing gambling.
But isn't the point of the Provably Fair system just to know for sure that everything is fair? As I understand it, any player with average technical knowledge/ability can check the fairness of the game on the history of their bets. I read that there are certain nuances and "loopholes" but in general the system works like this.
I think the gambling place that provides that sentence can be used to make players more sure about the betting system that is being carried out because usually when there is no writing like that the players think that the gambling place is manipulating the game, maybe that's what I know from the gambling place circulating at this time.