Conservation of mass?
Do they understand that producing energy and fertilizer using the bodies of animals is less efficient than producing the same number of calories or mass of nitrates from plants+less-energy-than-is-required-to-raise-the-animal-in-question?
Sounds like my coworker who argued that veganism kills more animals that meat does, due to the pest control/elimination measures that might be necessary to protect crops. As if pest control isn't a thing in the meat industry from the feed crops, to the feed lots, to the barns to the slaughterhouses.