I don’t know but I would think it would be hard to water cut through a large stone. The force of the water would dissipate over distance and those stones are incredibly thick.

It’s a fascinating problem. I was wondering if they used water as a level. They could fill up a pool and use the water level to set a line across all the stones. They could cut away any parts above the water to make them all smooth and even. Who knows! Quite the mystery.