The point is to emphasize that there's more cows on this planet using more resources than all of the other animals combined (excluding fish and water mammals).

You could add all the squirrels, elephants, lions, cats, birds, all of those, and you're not even at a fraction of mass of the cows we grow.

Yeah that seems phrased wrong, but here's xkcd visual: https://xkcd.com/1338/