Countries within nation states are not uncommon. For example, the Kingdom of The Netherlands consists of multiple countries: The Netherlands, Curacao, Sint Maarten and Aruba. The same can be said for example for the United Kingdom.
If it has more than one distinct nation within it, it is a multinational state, not a nation-state. Nation-states have been the dominant (but not universal!) norm for so long that people use “nation-state” when they just mean “state”.
That was exactly my point.
There's an important distinction between "states" and "provinces" that people tend to forget. Provinces are subdivisions of a larger entity, usually a unitary State, and inherit their powers from it. States, on the other hand, grant a central arbiter powers via cession; almost exclusively in the form of a federation or confederation.