Not to nitpick here, but that's not exactly true. At least not in Vegas. Reel configurations in physical slots stay the same, but the par sheet changes based on electronic weighting which is not disclosed to the player. A reel with 5 single cherry instances on it can be weighted to come up cherry an extra 1 time in a thousand, if they want it to. That's how they adjust RTP without changing the pay table.

Now it is true that a slot with a 1-to-1 instance to weight ratio of symbols on all reels -- or a game like VP where the deck odds are known -- can definitely be proven fair if the pay table and outcome are made known to the player. But it's worth pointing out that all that is needed to make the game provably fair is a pay table and raw data to conduct a statistical analysis. If you're providing that to the players, then actually giving cryptographic proof is kind of a pointless extra. It may make some players feel better, and I have no problem with that, but like you said once the player has the actual RTP values, the reel configs or basic odds of the game, and the outcomes of all the play sessions, that's already enough to prove whether the game is fair or show if it's rigged.

Of course a house could manipulate those figures. That's what licensing jurisdictions are supposed to audit, and why most reputable casinos operate out of Malta, Curacao, etc. It's worth pointing out that a "provably fair" casino using two keys could also cheat by discarding a hash that was unfavorable to the house without the player ever knowing about it. Unless all the stats are made visible at the end of the day, there's no way to know whether a skew like that is happening.