Why and how the text could be Bavarian

[davidma]

Did Bavaria exist then?

Of course. Here is a map of the Duchy of Bavaria.
Around 1400, Padua (Cimbrian) was still German-speaking. Apart from the city-state of Venice, as it was an island.

Padova and the neighbouring cities such as Verona were predominantly Italian speaking by Dante's time (circa 1305). The Da Carrara signoria was founded in 1318 and was very similar to the Dalla Scala government in Verona. Venice conquered most of Veneto in the first decade of the 1400s. I think the theory that it could be someone from Switzerland/Bavaria/Southern Germany still is quite likely, however he or she would've likely travelled to Italy rather than been an "italian" german native speaker