Which is part of the reason Bretonnia has a pretty large merchant class. To whom they sell said food/raw materials to import said cannons.It's from Knights of the Grail, by law the noblity are only able to tax things that have been witch have been grown or built by someone, like food or ore from mining. As the peasantry(specifically merchants) don't actually "make" money the nobility can't tax coin.
Also, if a commoner has a valuable skill(like literacy or cannon maintenance)? That guy is going to be more useful. Bretonnia is a nation of inequality, why would they keep all commoners equal when they could give the more useful ones more rights comparatively to their peers(to make their peers hate them, drawing attention away from themselves).
Besides, don't their cities contain a fair number of foreigners(who are also exempt from taxation on account of being foreigners, unless they lack proof of being foreigners)? They may very well have (disenfranchised former) Nulner engineers working for them.