The theory is that there was an older and more advanced civilisation that built them using more advanced techniques which are now lost.
And the Inca inherited pre-existing structures.
The Inca did do stonework of their own, but not close to the standard exhibited in this article.
Thats not a theory. Its the plot of a netflix entertainment show by famous sharlatan journalist (not a scientist, not an archeolog) Graham Hancock.
This doesn't pass the sniff test. How could a civilization supposedly far more advanced than the Inca vanish without leaving the Inca a single shred of evidence of their existence? No tools, no records, no memory at all? Oh, except the giant intricate stone structures they built. A coordinated conspiracy to claim their superior civilizational achievements as their own might be the only explanation of that and that's veering into comedy.