Not nonsense. Because did unions arise out of politics at the point of a gun, or did they arise in a capitalist system? The latter.
Capitalism is an economic theory, just like socialism, and just as socialism sounds really nice on paper, capitalism sounds really nice on paper. But pure capitalism would not work just like pure socialism would not work, because it is not an accurate reflection of how society works. Any problem that can be pointed out in socialism has an equal counterpoint of equal weight in capitalism. Unions are a patch to correct some of these holes in capitalism theory.