Is this the first time you've heard the term "nation-state" in cybersecurity context? Used as an adjective, "nation-state adversary" flows better/sounds cooler than "government-backed adversary" or "country-level adversary".

You may be right in poli-sci class, but that's just how it is in cybersecurity. To use your framework, it would be like me colloquially asking "do you have a car?" and someone responding "yeah, a Jeep Wrangler". Car = automobile. But I wouldn't be that casual when filling out my auto's title information.