No assumptions, it's in the paper you linked. They are basing on all land currently used for livestock farming being converted to human food crops. As the artic

No assumptions, it's in the paper you linked. They are basing on all land currently used for livestock farming being converted to human food crops. As the article itself notes that results in a significant over production of food: "The sheer amount of food we had available to eat increased by 23%." While interesting, that would not happen for long in the real world or prices would fall and farmers go bankrupt. Or perhaps a new export opportunity and some farmers elsewhere would go under.