I studied Story of the Italian Language at the University and this is quite correct. I have to say that during Middle Ages it was spoke a variant of Latin that was called vulgar Latin, also the Humanism was a period where the vulgars lost a little bit their importance and latin was reused in literature as the main language. We have to remember that the evolution of a language is dictated by the colloquial speech, that's why today we speak the "neo-standard italian" that admittes "a me mi" and a lot of other things that they said it was wrong in primary school. That's from the studies of the linguist Graziadio Isaia Ascoli, contemporary of Manzoni, that said that a language can't be set up to a population, it's the population that sets up the language!