The argument is that so much (animal or human) suffering has gone on already (or could be going on or is going on concomitantly), one more animal won't really add any additional suffering and therefore at any given meal there's no reason to not order an animal product. Examples include:
This is called "moral nihilism" because ultimately this position is that all morality is inherently meaningless.
If you do anything ever, and you live in a world where any significant amount of suffering ever exists, now everything ever is morally justified, because you can always deflect to that. "So many cars get stolen, what's another one?" "So many people die each year, what's another one [pulls out a gun]?" or even "So many people die each year, why not steal a dollar?". This is an obviously psychopathic view of the world.
Carnists claiming this also still would feel recoil in horror at taking a large knife and deforming a cute little dog for instance. "What does it matter, so many dogs have birth defects, blindness, or missing limbs? This is just another one of them now." Carnists would still be calling for that guy's head even though they turn around and propose some version of moral nihilism as a justification for their own actions toward animals.
Stealing $5, murder, buying a vegan sandwich, and throwing a baby chick alive into a shredder can't all be "equally wrong" if the word "morality" is to mean anything. That's just absurd. You don't get away with just pooling together your causing suffering with others doing the same. This isn't a real argument.
In this case, nihilism is a way to sort of launder the ethical implications of all actions so that you can then come in to axiomatically define carnism as ok vis a vis a more intellectual "Might makes right" stance. And therefore all the same problems apply.