When we look at human prehistory we're talking about a small number of individuals living in a small area, so a single ancestral diet is a reasonable approximation. Foraging was indeed more efficient than hunting in terms of calories, and yes the majority of food consumed would not be meat - I certainly wouldn't advocate an all-meat diet. But obviously meat was a valuable or even vital source of some nutrients (the very fact that humans continued to hunt when foraging is more calorie-efficient suggests that meat was necessary in some way), and I find it hard to believe that a diet that eliminates meat entirely could be healthy.