Petrarch, d. 1374, wrote that "This plague [firearms] was only lately so rare as to be looked on as a great miracle; now, so easily taught the very worst matters are to human minds, it has become as common as any other kind of weapon." As the Hussite Wars didn't begin until 1419, and artillery had been in use for about a century by then, it's unlikely that people in general wouldn't know what firearms were or thought firearms operated by magic. In any case, firearms were used by both sides. Duke Albrecht V of Austria in 1421 ordered that for every 20 soldiers there should be 3 handgunners and 8 crossbows. So while handguns weren't the most common weapon, they weren't uncommon either.

Source: Medieval Handgonnes, Sean McLachlan