[nolverostae]

I guess you're assuming this is about the weight of food they are eating. Why?

What they're actually doing is measuring the isotopes in the carbon of the bodies of the human remains. You don't get any carbon in your body from the water or from the fiber in plants.

So your estimate correction needs to go the other way - the study says that > 80% of carbon in the bodies of early human foragers comes from plants. That means that also > 80% of the caloric intake comes from plants. Or alternatively, it would mean that > 95% of what they ate (by weight) was plants.