Grass fed livestock is indeed a large portion of the world's meat consumption. Places like Argentina, Brazil, New Zealand, England, Texas have a great deal of cows, sheep, and goats walking the land and not fed through a trough. Please account those in your calculations of what it would take to replace a cow with a pile of corn.
Labelled grass fed beef that has not seen a feedlot is under 1% of the US market. It is also a dubious proposition that grass fed beef has any environmental advantages. At best it will find a niche as a luxury product as meat is replaced by plant-based foods.
Even in Argentina, the amount of beef that is purely grass-fed has dwindled. The popular image of standard Argentinian beef coming from bulls roaming the pampas has been obsolete for at least a decade now. Argentinian beefs are now fed maize etc. on feedlots like in most other developed countries.
Feedlot cattle more than double in weight in under 200 days. Beef cattle are slaughtered at under 24 months. How could 93% of a feedlot animal's diet be grazed grass?
The numbers there show the percentage of potentially human palatable food that is consumed by cattle, namely corn.
As the article points out, byproducts such as alcohol production waste are used in feedlot feed. This very much does not mean that a high percentage of feed is grazed grass.
If you have ever made the mistake of trying to eat feed corn, you know that the real world amount of human palatable food fed to cattle is about zero. But that does not matter.
The reason it does not matter is that the land, fuel, water, and capital equipment that goes into making feed for cattle is still entailed in feeding cattle, for about half the weight of each steer.
Tl;dr: the article is not apropos.
EDIT: I should clarify that I do not mean that feed amounts to half the weight of the steer. It takes 6kg of feed to add 1kg to the weight of a steer. Feedlot cows consume about three times in feed what they weigh on the way to slaughter.