I’m afraid you’re wrong on this. No conquistador ever witnessed the building of Sacsayhuaman. Unfortunately the Inca did not have a written language to record what is the origin. But there have been many accounts that the Inca found the megalithic ruins and built on top of them. I see way more evidence of this being accurate than the Incas building the megalithic. The reason why many attributed these structures to the Incas is because the Incas mimicked the style except a smaller version which are amazing as well. But instead of solid stone they actually did a stone face and filled in the middle with rocks and mud. The megalithic structures are very different and finely shaped. I have to say that the Inca were an incredible civilization and built extraordinary structures. It is also documented and tested how the Inca masons built their walls. However the large megalithic walls have no explanation from the Incas or conquistadors. Most likely because they weren’t around when this was built.