But is he wrong in that whatever thing comes out as nationalism that for lack of a better term he calls tribalism and nationalism is actually universal to the human experience and therefore if we apply the logic of why nationalism to separate historical instances we can come up with the same us-vs-them, united by common identity interplay that is effectively going to have the same composition of social interplay, just under different names (i.e. consistently analogous across time, space, culture, etc).
I mean, I think so, but what Terx is asking of you is why that is so. In so far as you're trying to be helpful to him, and not just to yourself or the audience at large, you're offering him the tools to figure that out.
But telling him why that is so, at this point, would be more resourceful. In fact, he could, perhaps analogize the specific argument into figuring out the tools for future examples.
Another way of putting it is, is the root of nationalism a pervasive and meaningful piece of trans-historical human nature that makes nationalism an appropriate word, pseudo-akin to saying Xerox instead of photocopy? Because that's what, I think, Terx has been hoping you would address.