I think it was something along the line of their stonework seeming almost too good to be true. Historians apparently dismissed the idea, but some engineers insisted that it seems near impossible to shape (some specific types of) rock as perfect as they supposedly did without precision tooling. There are explanations historians came up with, but apparently these were dismissed by the engineers and no one replicated and mastered the proposed methods.

But maybe the Egyptians were just amazing craftsmen that invested crazy amounts of ressources/time into a very intricate manufacturing process. I'm no expert on these things. I just like the idea that they found a random Pyramid in Egypt and spend the rest of their days building geometric shapes in the desert, because they thought it was pretty neat 😄