The claim here is that totalitarianism won't stop carnism, therefore it doesn't make sense to promote veganism because you'll never make it illegal, or if you do, it'll just make a meat black market or something.
The ethics of veganism stands and falls on the merits of the arguments, not based on some sort of meta-argument about how the legality of it all will play out in the future. If you think it's wrong to slit the throat of an animal for your sensory pleasure, then it's wrong based on the arguments against doing so. The legality of how it will all play out in the distant future is an irrelevant fact.
Does the fact that people still steal cars invalidate arguments that stealing cars is unethical?