If I had to take a guess, I would say he heard it in a lecture by Prof. Alessandro Barbero, same as I did :)
But I think the source is this: https://en.wikipedia.org/wiki/De_vulgari_eloquentia
In the Italian Wiki page, the "constructed nature" of latin is hinted at; it doesn't seem to be present in the English wiki.
Update: It's indeed in the book, at the end of the 1st chapter of the 1st book:
3 There also exists another kind of language, at one remove from us, which the Romans called gramatica [grammar]. The Greeks and some - but not all - other peoples also have this secondary kind of language. Few, however, achieve complete fluency in it, since knowledge of its rules and theory can only be developed through dedication to a lengthy course of study
4 Of these two kinds of language, the more noble is the vernacular: first, because it was the language originally used by the human race; second, because the whole world employs it, though with different pronunciations and using different words; and third because it is natural to us, while the other is, in contrast, artificial.
Here, vernacular refers to "italian" or whatever dialect, while "gramatica" is latin - the artificial one :)
