It isnt.
It requires statistical jiggery pokery to make it stack up, e.g:
Using carbon/1 element as the only measure - stupid. Works on paper, but defies common sense and fails to take in other factors.
More 'efficient' to feed people crops rather than animals - this relies on the assumption that aninals eat human grade feed, when in fact they are for the most part hoovering up byproducts from our food production. It also ignores the impact of ploughing, diesel etc., all sacrificed for the great mythical god of 'efficiency'.
The rest relies on sheer ignorance of the food system, like the comment upthread about aninals needing to eat crops grown elsewhere, especially in winter. Nope, they can eat the same locally grown grass preserved for winter, or the same byproducts generates by our year-round eating habits.
When you think about it seriously, it all breaks down: in a fossil fuel free age, what were we eating? Meat, and plenty of it. Our ancestors didnt have shit to waste by frittering it away on aninals, so thet must have had good reason (namely, it being efficient and easier than grafting over crops). Now we have abundant fossil fuels, we live mostly on grain facilitated by that. Toast, granola, porridge, followed by cakes, biscuits or pastries, wraps and sandwiches for lunch, then whatever accompanied by chips, rice, pasta etc.
Animals can exist in natural habitats - grass and woodland. There are no natural turnip fields. Animals are used to manage habitats for wildlife. There are no 'conservation potatoes'. Seeds grown for birds are there because we stole grass and woodland for crop production, and humans dont eat them: it's a sop.