I don't think the premise is flawed. If plants have a 60x range in input (as proxied by cost) per calorie provided, then animals can consume plants at the top e

I don't think the premise is flawed. If plants have a 60x range in input (as proxied by cost) per calorie provided, then animals can consume plants at the top end of that range and still provide more calories per input than plants at the low end. 60x is a huge range. With regard to the methane from cows concern: just eat chicken or pork. Chicken is probably the most environmentally friendly meat (in terms of calories provided per input).