Well it is well known that even before Christ that there were many mathematicians, philosophers and scholars that were able to amaze us all with such inventions and calculations, in fact just by using the angle of the sun someone in Egypt was remarkably close to the distance around the Earth, if I’m remembering that correctly. Again with the other pyramids, even though not near as huge they were extremely accurate with every block placed. Plus with lack of documentation in South America at the time it is very possible that they could have had a similar great mathematician or engineer that could have done it, by no means would it be impossible if you had enough materials and lives to spare.