Nowhere in its constitution is defined like that, it’s not a confederation of nations but a parliamentarian monarchy that respects regional cultural differences.
Many scholars, politicians, and citizens, even excluding people outside of the Overton window, describe Spain as containing several nations. At the very least Catalonia, the Basque Country / Euskal Herria, and Galicia are widely (but not unanimously) considered to have the historic, cultural, social characteristics that would make them nations. The spanish constitution is a document of compromise, born of a very special time in history; whatever it says about what Spain is, we don't have to accept it.
(Biases disclaimer: I'm Spanish, specifically castilian; I don't support Catalonian independence, and I dislike all nationalisms; all nations are more like big balls of wibbly wobbly... nation-y wimey... stuff)
This piece of news doesn't yet show up on the spanish newspapers I trust; I'm really curious to see if it makes the news. I'll be extremely concern if, as I fear, if doesn't.
We are very misunderstood, for sure.