Over many generations by natural selection, populations adapt to their food sources. Those living in a culture that eats its traditional diet will be eating the correct food for their metabolic type and individual physiology due to their genes being adapted to that food source. So you have Japanese being quite all right with rice while many Caucasians, while most likely having more "tolerance" to grains than Grok would have, still do not do extremely well on a high-carb diet. Conversely, many hunter-gatherer tribes live very long and healthy lives eating large amounts of meat and vegetables without agriculture and neolithic foods, but start feeding them a Japanese diet and they do terribly.
Nobody is saying that some people can't be very healthy on a non-primal diet. What we are saying is that for a good many of us whose genes do not have such great adaptations to neolithic food, the paleolithic diet and PB work extremely well. Look around this forum and see people thriving and generally stoked about their health and quality of life.
Hrm. Do we have any Chinese or Japanese around here? I would like to know how successful they are on the PB as opposed to those from Europe.