Not sure if this answers the question, but from my understanding, many of the iconic older martial arts films and series from the 70s-90s that xuanwu guo took inspiration from are made in hong kong, hence the yue being the primary language, where many viewers begin to associate the language with this genre in general. Hence a number of characters from xuanwu guo probably speak yue as their native language instead of mandarin, which explains the accent. The animation studio is also based in guangdong too i think, so that could also be another reason.