The idea here is that the act of harvesting plants kills animals by harvesting maceration, pest removal, etc., and therefore:
Humans die in the production of crops. Could Jeffrey Dahmer use this for his defense of killing and eating humans? So then why does this defense work for animals and not humans?
As this article notes, "Alternative tillage practices, indoor farming and rodent contraceptives are existing agricultural practices that have the potential to reduce field animal deaths, and there are others that we might eventually develop. However, none of these practices have received any attention in the conversation about field animal mortalities"
If you went to a farmer now and said "Hey, let's work on crop deaths" they'd tell you to get off their property because they're busy on the other side of the farm growing animals to get stabbed in the throat. So as long as we're stabbing animals needlessly how on earth are we going to get the will together to eliminate all animal deaths from food? So then this argument is just "We've been farming in a way that harms animals, therefore we should continue to harm animals because that's how we farm. And my argument against change is that farming, as we currently do it, hurts animals."
Any problem that is caused by farming crops is only going to be magnified by then feeding those crops to animals and reaping even fewer calories and nutrients. Shouldn't you still reduce the number of animals killed by just ordering something off the menu? That is to say "I won't move an inch to reduce the deaths by 99% if you don't move a mile to get that last 1%". Factory farming still remains unethical.
So unless one removes all non-pasture-raised animal products from their diet, this isn't the justification for one's carnism. It is a plan that avoids going 100% vegan by going 92%+ vegan. Eating a calorie of chicken still causes 2-3 orders of magnitude more deaths than grains, again, given our current technology and general lack of regard for animals and general will to reduce that number. All of these arguments therefore apply.