The carnist here claims that veganism is a necessary but not sufficient condition for completely eliminating global problem X, and therefore cannot properly be regarded as the cause of X. E.g.
"Education isn't enough; students also need to put it into practice" cannot be followed up with "Therefore education is always pointless". The fact that two things need to change concomitantly isn't an argument against changing either one of those two things. Do you quit whenever you hit a roadblock because there might be a second one as well? "Welp, if we all stop torturing animals for our own entertainment, we would need to also figure out redistribute this new excess of food. I guess I need to keep torturing them." This is ridiculous. You can always point to the other thing as being the cause.
Carnism vacuums up food supplies in third-world countries, so why then would networks exist to distribute food that doesn't exist? And how would increasing the supply not decrease the price just according to basic supply and demand? The torture of animals is still the fundamental reason that this food supply doesn't exist, and stopping torturing animals is still the easiest way to create a surplus to alleviate the food shortages.
Sure, if everyone in America went vegan, that might not solve the food shortages that happen in North Korea, for instance, because it's a completely different set of causes. That's not a good argument for continuing to imprison and slaughter animals, namely that you can't solve some other problem in its absolute entirely given all the conditions that would be at least partially alleviated by ending this practice.