The "giant if" you mention is stated explicitly in the section you quote: "More precisely then, ethical consequentialist vegetarians believe that farmed animals

The "giant if" you mention is stated explicitly in the section you quote: "More precisely then, ethical consequentialist vegetarians believe that farmed animals have lives so bad they are not worth living, so that it is better for them not to come into existence. Vegetarians reduce the demand for meat, so that farmers will breed fewer animals, preventing the existence of additional animals. [...] I will argue that if vegetarians were to apply this principle consistently, the suffering of wild animals would dominate their concerns, and would plausibly lead them to support reducing the number of wild animals, for instance through habitat destruction or sterilisation."