Nutrition science is quite complex and much of the published literature is simply junk. A lot of what we thought we knew a few decades ago turned out to be wrong. So it's not entirely unreasonable to ignore the science altogether and just eat the same foods that your ancestors ate before agriculture. It might not be optimal from a nutrition standpoint, but it's unlikely to be too far wrong.

Of course most "paleo" nutritionists are scammers pushing cookbooks and dubious nutritional supplements.