Cordain confuses the issue through his choice of words. Saying grass fed animals are low fat is bullshit. Law fat compared to what? Compared to grain fed animals then certainly. Compared to most foods CW tells us are healthy then grass fed animals are high in fat.
His second fallacy is that he fails to ask whether the whole animal was ate all the time, or only when necessary. Kangaroos for example are low fat animals, but the natives only ate the fattiest parts, making their diet high fat despite their source low fat, by Cordain's poorly worded logic.