Not true. American farmers and agribusinesses have increased average yields by a quarter or more in the past 15 years or so thanks to massive investment in seed and precision production technologies. Yields are expected to continue to increase and may double within the next 20 years. This investment would not have happened without the demand pull of the ethanol buildout. You cannot look at the current production base, take it as a given, and forget how and why it was built.
Also remember that, in terms of nutritional value, a third of the corn that goes to ethanol returns to the feed market as DDGS.