This isn't an appeal to empathy, it's a rational argument based on the mental capacity of the animal and our existing standards for ethics. An empathetic appeal would be to say, "when the octopus suffers, you feel sad, therefore because you feel sad you shouldn't cause it harm". That's not the vegan moral case. Your feelings don't matter. Speciesism is illogical and requires empathy or lack thereof. You don't feel bad when you cause the octopus harm instead of a human, therefore the octopus doesn't matter. Logically, the vegan is on perfectly solid ground.
You have it the exact wrong way around. In social decisions empathy weighs very highly and logic counts for very little, which is why tribalism is such a huge driver of human behavior, even when it causes many more living beings harm than it helps, as it does during war.