He's not exaggerating much. Even when Cahokia scholars agree, they tend to frame their positions so it seems like they're disagreeing—but there are points of general consensus. Everyone agrees that Cahokia developed quickly a couple centuries after corn became an important part of the local diet, that it drew together people from the American Bottom, and that it dwarfed other Mississippian communities in size and scope. The battle lines tend to form along the questions of how populous it was, how centralized its political authority and economic organization were, and the nature and extent of its reach and influence.
At one extreme you have descriptions of Cahokia as a "theater of power," a hegemonic empire sustained by force that reached deep into the Mississippian world and perhaps connected to Mesoamerican civilizations such as the Maya or Toltec. At the other extreme you have characterizations of Cahokia as little more than an especially large Mississippian town whose residents had a talent for making big piles of dirt. But as usual, most of the action happens in the middle area between those poles.
Right now the discussion is being spearheaded by Tim Pauketat at the University of Illinois, who with his colleague Tom Emerson argues that Cahokia’s big bang was the product of a visionary moment: A leader, prophet, or group cast a vision for a new way of living that attracted people from far and near, creating a rapidly expanding cultural movement.