According to this analysis (probably a biased/motivated source but not one worth lying about actual output for its members to make economic planning around), each bushel produces 2.9 gallons of ethanol, 14.5 lbs of distillers' grain, and 0.9 lbs of corn oil.

Distillers grain is a pretty useful animal feed, with about the same amount of calories, and a higher protein content, than regular corn per pound.

So if a 54 lb bushel of corn that has been used for ethanol production still has about 14-20 lbs of grain equivalent (depending on how important that higher protein content is for the animals being fed), then some percentage of that corn being used for ethanol should still be counted towards animal feed. Depending on how you want to account for the oil, too, there's probably some feed value there, too.

first, this relies on poore nemecek 2018, which used poor methodology, so you shouldn't believe the study you cited.

but beyond that, much of that farmland is grazing land. that's not 80% of agriculture, it's 80% of agricultural land.