I agree that the claim about "food value" is in need of rewording, but it's context makes it clear that we're talking the total number of meals that the ecology

I agree that the claim about "food value" is in need of rewording, but it's context makes it clear that we're talking the total number of meals that the ecology of the region can provide, and not the percentage of any given meal that happens to be bioavailable. It's not about running faster than your vegetarian rival, it's about whether your tribe collapses because there isn't enough food. Yeah, not all plant matter is food for humans, but if you only had, say, 100 square miles, and you wanted it to support as many humans as possible, how would you allocate resources between hunting and gathering? The point is not that one choice is better than the other, it's that those who happened to make the right choice at the right time were the ones that survived.