This makes a kind of sense for me. There is a large mound city that is similar in structure to the Incan/Mayan city setups with a central mound/pyramid in southern Illinois.
To me it would make sense that the two groups came back together at a just before or after that time and that the knowledge of that type of city planning then headed south.
It could be that those who rejected it headed NE....similar to most liberals living in cities now and most conservatives living in the suburbs and rural areas.
The Inca civilization in South America had incredible infrastructure. I have hiked on their trails during mountain climbing trips.
I have also been to the National Museum of Anthropology in Mexico City. The Aztec displays were amazing. They had human sacrifice problems, but had incredible artifacts that were left behind. I must have used 10 rolls of film at the museum.
I went on to climb Ixtachupatetl, Popocatepetl and Orazaba, high altitude volcanoes and the 7th, 5th and 3rd highest peaks in North America. Popocatepetl was blowing steam when I was there, but then it started real eruptions and has been closed to climbing for maybe 20 years.
That would be well after the period they are talking about.
Cahokia flourished in the Middle Ages...around 1200-1400 if I remember right.