I think it to be just a myth. But I do know that De Soto died on the banks of the Mississippi (of fever) with his troop in disarray. No one knows exactly where he died or was buried. In fact, it's thought that his loyal followers may have put his body in weighted blankets and sunk it in the river. Which would be fitting, somehow. He was the first European to record the Mississippi.
As far as the population density of the valley, it was really quite dense. They had a well structured agricultural basis and lots of craftsmen and tradesmen living in cities and villages. They had a city larger than London (the largest city in Europe) for a couple of hundred years. 6 million would have been roughly 70% of the population.
Thank you for the wonderful comment.