That is very fascinating. Do you have some source on this that you could share? IIRC Dante wrote in vernacular Italian which was uncommon at the time, presumably to make his texts more approachable by common people?

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 :)