Not read too far into it yet but he does seem to back up everything he says with scientific evidence and names the author of his source .
To be honest the whole idea is what i pretty much do already , although i thought adding pasta and grain to my diet was a healthy option until reading this book . I dont eat a lot of that anyway .
I'm not sure how it can be a fad diet as it just says only eat what you can forage or hunt from a man that has seen loads of fad diets to get very close to the top of his sport and suffered because of fad diets / performance diets .
By all means prove me wrong, but hunter-gathers would almost certainly collect and eat grains and legumes, they were after all, gatherers as well as hunters. There's plenty of anthropological and archaeological evidence to support this, just google it. However a shift to horticulturalism will have allowed humans to massively increase the proportions of cereals and grains to the detriment of other food intakes. They would no longer have to roam so far, as easy calories were on the doorstep. Humans are lazy, they want to conserve resources. If your pantry is stocked with grain for baking bread, suddenly that 2-5 hour trek from the settlement to collect leafy greens looks like a lot of hard work... so to a point I agree with the book, grains are bad for humans... but only humans without self control and knowledge of what it takes to form a good diet. If you only eat grains, and even worse, processed grains, you're definitely going to get sick, or suffer some forms of malnutrition.
However it's not cereals/grains that make people sick , it's the lack of varied intake. The book states you don't need grains, therefore you shouldn't really eat much of them, and you have fallen for the con, grains are not unhealthy
per se. Grains actually are amazing, as they are a fantastic energy source, and wholegrains and cereals keep you full for a long time, the problem in modern western diets stems from eating too many refined grains, which strip out nutritional value and leave behind just the carbs. Anyone who exercises a lot certainly benefits from eat grains, sure, you don't NEED grains to exercise, but it sure doesn't hurt to eat them either, so long as you don't overdo it. The same stands for other foods. If you only eat leafy greens, or meat, or eggs... you're going to get sick. Sedentary desk jockeys however, really ought not to eat too many refined grains for obvious reasons. Let's not forget there is good reason porridge before a 60 mile ride is popular.
IIRC, this book, as well as other well known paleo diets deny that grains and legumes will have been consumed and important to hunter-gatherers (ideology), and the primal blueprint website goes as far as saying that grains are pointless.
This is the crux of it, shunning entire food groups on a whim because of a belief that hunter-gatherers didn't need cereals, grains or legumes, therefore we should avoid them makes it a fad diet in my book.
Now our ancient hunter gatherers won't have eaten as much grain as the typical westerner does today, and what they did eat likely wasn't refined (white flour, pasta, cake etc). The Mediterranean diet, widely regarded as one of the healthiest in Europe includes lots of grains, cereals & legumes and oils. So grains/legumes are not unhealthy, but should be eaten in moderation and according to your personal requirements.