What conclusions did historians draw about the incas after the discovery of machu picchu

I'd say that the conclusion historians point to about Machu Picchu is that the Incans were skilled engineers.
There's no evidence that the Mayans or the Aztecs influenced them in any way, and the Incans did have an organized system of communication using knots of tied rope to keep track of everything from inventory to history. Unfortunately, no one today knows how to interpret these knots.

The answer should be :  The Incas were skilled engineers.
Hope this helps !

Photon