It's gradually becoming more and more clear that the Amazon basin was more densely inhabited than previously imagined, and that such populations were much more settled and agrarian than the scattered survivors of the post-Columbian exchange.
To be fair though, believing jungle agriculture could not support such populations was perfectly reasonable given the typical soils in those places. They're low in nutrients (and thus easily exhausted), and the rains help wash out much of what does accumulate. Slash-and-burn operations have to keep moving to new areas precisely in order to sustain productivity with new fields. Given how hard it can be even with modern technology, crops, and animals, it didn't appear possible to feed a population efficiently enough with pre-modern tools and techniques to sustain a high population density.
And all that is still absolutely true. Over time however, archeologists have managed to show that you can change the available soil into something intensely fertile that both sustains its nutrients and regenerates itself once established. This soil the pre-Columbian inhabitants created was called
Terra Preta, and was an oddity given how when Europeans first discovered it, disease had already wiped out the communities that produced it, and the jungle easily swallowed those fields back up in the intervening time.