I think it's more complicated than this, at least for land mammals. The fact is we're not out hunting land mammals; we're raising them for slaughter in a controlled way. If we ate less meat, fewer animals would be born in the first place (since we strictly control their numbers). We can't really count an animal not ever born as a 'saved' animal. However, the animal never being born in the first place is the best possible scenario for our resources and the environment. So, really, while I question the author's math somewhat, I think he is actually under-representing the positive effects of shying away from meat.
> We can't really count an animal not ever born as a 'saved' animal. That depends upon your goals. If your goal is to reduce the number of animals that are slaughtered, then that might be a standpoint for ethical vegetarianism. If your goal is to have more animals living at any given time, then that isn't a good reason (because it can be reasonably assumed that economic pressure by vegetarianism results in a lower breeding rate). In short: ethical-vegetarianism is complicated.