This would be true if livestock ate human food. As it is they mostly eat things we can't, like grass, on land that is poor for agriculture. Feed corn usually just fattens them up there at the end, and I'm all for minimizing that disgusting process.
The idea that most of the meat we eat was once animals grazing on grass has sadly been untrue for a very long time.
Most cows, pigs, and chickens are raised in tightly-packed conditions, and fed things you really don't want to know about. There are grains in there, but a lot of nasty things get mixed in with them.
A lot of land that could go to raising human food instead goes to growing soybeans, corn and grain that are fed to livestock. You're right that we can't just divert the grains from animals to people, but if that land were used to raise human instead of animal food, we'd be able to produce 16x as much food by volume as we do with meat.
