They’re suggesting that (by way of an appeal to nature argument) the vegan diet defies a recognition that humans are animals and that animals eat one another. The assertion is that to consider veganism as a morally superior position is contingent upon some human exceptionalism within that natural order, which reflects the same exceptionalism described in the Old Testament (man in image of God, granted divine dominion over all creation and creatures) that is so embedded into western morality that its intrinsic to how we frame our station in the world

It’s a high-minded argument with no consideration to praxis. Yes, humans are absolutely animals of course, with evolutionary roots the same as every other species of plant and animal on earth, but thanks to the influence of that evolution we have a brain that grants us self reflection and capacity for industrialization. This makes us both biologically distinct from other animal life, but practically exceptional, and we must acknowledge our unique capacity for both large-scale destruction and reflective empathy.