Same with modern French, which is basically Parisian, modern Spanish essentially Castillian. There is never an neutral language, it is linguistic supremacism one over the other. I absolutely respect Sicilians (and other regional groups) for resisting 'Florentine cultural imperialism'
Control over the Church was rarely as useful within Italy as outside of it. The papacy always had strong enemies on the peninsula. Much of its policy was dedicated to keep them divided and to ensure that those didn't ally with outside powers.
French was normalized as a written language around the great feudal courts of Northern France, at the time Paris wasn't particularly influential culturally. Parisian French was itself quite distinct from "government French" until recently.
My personal experience also corroborates this. In common usage, Castilian is the same language as Spanish and in fact I hear people from Spain refer to the language as Castilian, even when talking about the language as spoken in Latin America, regardless of the Academy's prescription.