That passage is based on the works of Henry F. Dobyns. In his 1983 book, Dobyns advocated for a 18 million strong pre-contact population in North America. Specifically, he gave an estimate of 5,250,000 people living in the Mississippi River valleys. This, according to Dobyns, amounted to a population density of 2.53 per km2.
Horcicultural peoples estimated to include 5,250,000 individuals lived there, averaging 2.53 persons per km2.
- Dobyns, Henry F., and William R. Swagerty. Their Number Become Thinned: Native American Population Dynamics in Eastern North America. Knoxville, TN: University of Tennessee, 1983. Page 42.
Dobyns arrived at his estimates by working from later figures, settlement sizes, epidemic losses and estimates of food production levels. A core idea behind these numbers is the theory that population expands to the maximum the environment can support. But the results are widely criticised.
Many scholars consider Dobyns's methodology based on the carrying capacity of the environment to be flawed and his population estimate to be far too high.
- Perdue, Theda, and Michael D. Green. The Columbia Guide to American Indians of the Southeast. Columbia University Press, 2001.
In contrast, Douglas H. Ubelaker in 1976 estimated a pre-contact population of about half a million in the Gulf of Mexico. Note that he used the geographic classification of Mooney, so the figure is not for an area identical to Dobyn's Mississippi River Valley. Nevertheless this gives you an idea of the huge disparity between these two sources.
Another estimate for the Southeast is that of Charles Hudson, who also criticised the steep decline population decline presented by Dobyns. Working backwords from Peter Wood's late 17th century figures, Hudson tentatively calculates a 1,294,000 pre-contact population.
Using early census, head counts, and estimates, Peter Wood has estimated that the total Indian population of the Southeast in 1685 was about 200,000 people. If the 6.47-to-1 rate of decline in Coosa may be applied across the board in the Southeast, perhaps we can use Wood's figure to estimate the sixteenth century population of the Southeast at 1,294,000
- Hudson, Charles. Knights of Spain, Warriors of the Sun: Hernando de Soto and the South's Ancient Chiefdoms. University of Georgia Press, 1998.
Wikipedia has this map for the area occupied by the Middle Mississippian and Plaquemine cultures. These two were primarily centred on the Mississippi embayment:
For an idea of where Dobyns lands on the spectrum of North American Indian population estimates, see the following chart:
(Source: Stuart, Paul. Nations Within a Nation: Historical Statistics of American Indians. VNR AG, 1987.)