I've often come across a concept in magic performance that what the performer is aiming at is for the only available explanation for what you see would take an amount of effort that you immediately discount because clearly nobody would put that much effort into making a ping pong ball disappear. There are two ways to make the ping pong ball disappear: either the performer is cheating somehow, or they did actually do it the obvious way and yes, they did put all that effort in.

This seems the same: the idea the people shaped these stones by hand seems so outrageously profligate with human exertion that you look for how they cheated. But the answer is that it's actually slightly less exertion than you think, multiplied across far more humans than you think, but yes, they did go the long way round.