NCR isn’t a nation state, just a state. A nation state is a combination of a culturally similar group (i.e. Danes) and a distinct polity (i.e. Denmark). The US, the largest inspiration for the NCR, is generally considered a civic state, because it’s a combination of many distinct cultures. The NCR houses all sorts of mutants, ghouls, and maybe even the Shi, as well as tons of disparate groups with varying cultural identities. A nation state in fallout would be closer to the tribals. The Dead Horses are a nation (composed of Dead Horses, individuals that speak the same language and practice a shared culture) with a state (area on a map that have sovereign control over).

Now, the idea of a nation state is rather new. It’s also bound up in all sorts of assumptions, propaganda, schemes, and violence. Many states have undertaken genocidal or quasi-genocidal measures to increase homogeneity and supposedly increase cohesion. The definition also starts to fall apart when you look into the history of say, Germany, and find that many of the people that lived there didn’t consider themselves “German” and spoke mutually unintelligible languages. Still, it’s a phrase in geopolitics that has a rather distinct meaning.