Some useful comments on slash and char.
There was a very interesting story on public television a few months ago. It seems that when Pizarro and his men invaded Peru in the 1500's, he ran into some trouble and sent a group of men down the Amazon in boats to try to get some help from the Atlantic coast. One of the officers of this expedition wrote in great detail about this very difficult trip.
The thing that amazed me was that there were very large quantities of people living rather well along the shore. I could be wrong, but I think the figure several million was mentioned in the program. Apparently, Pizarro's men were treated well by some of these people. Then of course the Old World gave the New World the gift of Measles and Smallpox and all these towns and cities disappeared. Now in the jungles of the Amazon, the only natives are small very hidden primative tribes. Probably they survived because of their isolation.
In all fairness I should mention that the New World gave the Old World the gift of Syphilis (known in those days as the Pox), but I still think the Americans got the worst end of the deal. At any rate these soils are probably a remnant of what was once a very successful civilization.
I have read many accounts of early Spanish explorations and can't recall anything this silly, particularly from published versions in the original Spanish journals and diaries.
It's hard to imagine the likelihood of a group focused on reaching the coast for help stopping to make a census involving several million people even assuming such a thing were practicable.
Most likely this historical invention will be repeated over and over until its nonsensical presmise is assumed to be genuine. The artful interpretation of someone not versed in the practicalities of a large scale census in a wild jungle setting.