What if you were on a desert island?
Nearly every ethical framework permits survival exceptions. Almost nobody raising this is choosing between a pig and starvation while standing in a supermarket.
If you would kill and eat an animal to stay alive on a desert island, then you already accept that human needs can outweigh an animal's life, and the difference between you and a meat eater is only where to draw the line. A rule that gives way the moment survival is at stake shows that animal interests do not count as much as human ones, so it is fair to ask why ordinary human nutrition, health and wellbeing should not count as well. Veganism presents itself as a firm moral principle, but the island case shows it is really a matter of degree, and matters of degree are open to reasonable disagreement.
The scenario is conceded readily: in genuine survival conditions, with no alternative, eating an animal is defensible on almost any account of ethics. Veganism as standardly defined is explicitly limited to what is possible and practicable, which is precisely the clause that handles this case.
Having conceded it, nothing follows about the available choices. The hypothetical establishes that the prohibition is not absolute, which no one claimed it was. It leaves entirely untouched the question of what someone with a full supermarket should do.
Introducing a point that is not relevant to the question under discussion, drawing attention away from it rather than answering it.
How it applies here: Shifts from the choices actually in front of the person to an emergency that neither party is in, and which both would resolve the same way.
Replacing someone's actual position with a weaker, distorted version, then refuting that instead. The original argument is left untouched.
How it applies here: Treats veganism as an absolute prohibition holding in all circumstances, when its standard formulation is explicitly qualified by what is possible and practicable.