So what if Orellana had found an empire on the scale of Classical Tikal, or Inca Empire?
He may very well have found that, If not more. The Terra Prieta mounds in Amazonia are extensive. His descriptions speak of constant settlement along the river.
The evidence is for a substantial population pre "Columbian Exchange".
The Mounds would have supported significant agriculture independent of the Rain Forest ecology.
The issue is that the Amazonian culture, as far as I can tell, was impoverished by a lack of Minerals. They were great potters, but had no metal to speak of.
Additionally, there seems to have been more than one Amazonian culture. Some were in the upper reaches of the watershed and some were in the Delta and some were in between.
The environment there does not treat artifacts with kindness, so our knowledge is sketchy at best.
The book 1491 is a great start to investigating this possibility