just a point, isospin. In modern terms, the W boson *IS* the weak interaction. The W comes from "Weak". Try to keep focused in post- QCD models.
Of course in the 1950s the terms weak and strong, both, were standing for different objects that nowadays.
So if you tell that something decays to a W boson, and its mass is less than the mass of the W, it is the same that telling that it decays via the Weak interaction.