brute would like to contrast this with "30-50% of all humans in the US are considered overweight/obese", "As of 2015, an estimated 415 million people had diabetes worldwide, with type 2 DM making up about 90% of the cases", and so on.
while humans can survive eating pretty much anything, clearly, they do not thrive in the long term in a multitude of environments.
brute's diet is only "extreme" if compared with the current SAD, which is certainly "extreme" in terms of outcomes - see the facts above.
the idea that humans should eat 80% of their food as carbs is "extreme" in brute's view, whereas an all-meat diet he considers pretty "normal". this is because brute has recalibrated his "normal". the regular "normal" isn't working, and shunning "extreme" things is basically appeal to authority of scary words.
the claim that humans "need to eat lots of different things to meet their requirements" is demonstrably false for many definitions of "lots". brute has eaten this way for over a year, and similarly for years before that. brute eats 2-5 meals over and over again, almost always with the exact same ingredients. clearly, he doesn't need more variety, at least to survive, have good blood lipids, be active, and so on.