The film states that (the Peruvian Indians) could not see the Spanish ships when they first arrived because they had no prior knowledge of any such things existing. They could see the ripples in the ocean or something like that but at first not the ships. Based on what?
What are boats made of? Did the Indians not know about "wood" or "cloth"? Maybe...MAYBE...this theory could mean something IF say, Spaniards had arrived in say, a helicopter. That would probably freak the natives out...but no, cuz they'd probably be able to see and hear the choppers. I don't know.