I suppose this would pertain to medieval vocal music; would have this been something his father would have taught him about, or did his musical knowledge/training extend only to around the Baroque era (we know he admired Bach, for one)? Was medieval music even being studied in general during this period (late 18th century)?
Would they have had working knowledge of the theories medieval composers of chant/discant were working with? No, the terminology, techniques, standards, and notation were quite distinct.
However, we do know that Mozart and Beethoven both wrote masses, which are musical works that have their basis in the liturgy. I will admit that I cannot remember if at this time the mass was intended for church performance as a part of the liturgy, or if was thoroughly its own genre. It certainly was a genre distinct from the service of the liturgy by the time Verdi wrote his. We also know that around their time that Berlin Singakademie was starting to revive older works of music (but usually in private settings) and the German speaking printing industry was seeking to market a.specificallg German compositional art. Effectively, it is around the turn of the 18th century into the 19th century that people became more interested in performing old works.
Mozart quite likely heard some older music when visiting the Vatican, considering he transcribed a piece by Palestrina while he was there. Berlioz, who is a bit later in the timeline, quotes a very famous Dies Irae melody in his "symphony fantastique" and it was a favored melody of Liszt as well, although both are far after Mozart's time.
I do think it likely that Mozart heard older music, considering his touring and his prominence. He certainly would have heard some in the psalms of the Catholic liturgy, but it is unclear to me how influential this exposure was for Mozart. However, even if he heard chant or discant, his approaches to composition would have been radically different than any of what he heard, so any influence would be what some musicologists call "Schizophonic mimesis", in which the music is deprived of its original context (in this case, the practices contemporary of when the work was originated).
Tl;Dr he might have heard it, but he would not have studied it systemically compared to more recent works such as Palestrina. Even if he studied it systemically, it is unlikely he would be familiar with the theoretical underpinnings that informed medieval compositional practice, and it probably would not have been considered worthy of study compared to the works of Palestrina (a popular pedagogical model) or popular contemporary composers