In Civ 6 why is Mexico City still a city state when Astecs exist as a civilization and their first city is Tenocthitlan?

I thought they add and remove city states from the game to not duplicate the already existing cities of civilizations. My favorite city state Antitoch was removed like that when Byzantine was added as a civ in its own DLC. So I was surprised to see Mexico city in my game yesterday when my friend had picked the Aztecs, there were literally Mexico City city state and Tenocthitlan in the same game. Was that a bug or an oversight?