This argument would be interesting if the author wasn't an idiot and had a grasp of what sarcasm is. A: "All rodents are mammals, but a weasel isn't a rodent,

This argument would be interesting if the author wasn't an idiot and had a grasp of what sarcasm is. A: "All rodents are mammals, but a weasel isn't a rodent, so it can't be a mammal." B: "This does not logically follow. You evidently know nothing about logic." B's argument is still not ad hominem. Note that B directly engages A's argument: he is not attacking the person A instead of his argument. There is no indication that B thinks his subsequent attack on A strengthens his argument, or is a substitute for engaging with A's argument. Unless we have a good reason for thinking otherwise, we should assume it is just a sarcastic flourish. What a genius!