All that the provably fair thing proves is that there was no interference from the house to absolutely guarantee that the house would win any particular bet. The problem is that 'fair' is such an arbitrary term. In the long run, the player will always lose and of course sometimes it doesn't take that long.
No, it's really simple. It's fair because it does exactly what it claims to do.
But back on the 'provably fair' angle... people tend to not understand the "kelly" system or how that mixes with other factors rather it be bet size or any other multitude of factors that go into these algorithms designed to make the house profitable.
The only one who doesn't understand is you. None of those are an input to the provably fair system. In fact, the kelly only is used to determine max bet limits to prevent unacceptable risk