Corn has very shallow roots and aggressively pulls nitrogen out of the soil. Prairie plants are much more diverse than just grasses and many have very deep root networks extending down 6+ ft. Corn fields are nothing like a prairie habitat.

The area this article is talking about was oak savannah:

Within these oak savannas, which were interlaced with prairies, tree crowns covered between 10 percent and 30 percent of the ground. They were essentially a transition between the tight deciduous forests of the East and the fully open grasslands further west.