Did you read the next sentence after the two you quoted? The point is not how much carbon is stored in the prairie but how much it changes by each year.

The prairie does not accumulate more carbon each year; it is roughly carbon neutral as decaying matter balances new photosynthesis. The corn field has a large part of the biomass carbon removed from it each year. Within a few years the destruction of the prairie has been cancelled out.

This is true even for most woodland, although of course the break-even is much much longer. I'm not arguing in favor of cutting down first-growth forest to grow ethanol though, and I don't think that is the typical case in the United States.