This particular site is being built basically in the parking lot/backyard of a steel mill, so it sort of is brownfield. I have a hunch that “empty cornfield” is just the author using some artistic freedom instead of saying “an empty field in the Midwest”.
It was definitely farm fields growing crops, you can look in Google Earth and see for yourself.
Edit: Anywhere in the Midwest where there is a large expanse of flat ground will almost always be growing crops or have a town built on it. Undeveloped land is almost all hilly and forested or floodplain, nowhere where you could put a data center.
