Is there a counter-theory?

Pounding stone seems reasonable to me. Obviously I don't have any proof or even strong evidence but I saw a video that changed my perception of what is possible. It showed two old men making a millstone with hand tools: https://www.youtube.com/watch?v=lscs5ZgNQrE. The amount of labour involved and quality of the finished item was astonishing to me. Maybe you'll think that the hideous amount of labour needed to make a simple geometric shape makes you even more convinced the Inca has some other way to achieve their even harder task. But it is a fun video anyway.

Similarly astonishing to me is that Michelanglo's David was carved from a single piece of marble with a hammer and chisel. I mean, just look at it: https://en.wikipedia.org/wiki/David_(Michelangelo)