Momiji has appeared in the manga before the teacher, the sushi guy and the Akai father.We have enough new characters with the Nagano eye-patch guy, a teacher, a sushi guy, RUM, and the Akai family.
Momiji was already a Karuta player in the manga before the movie came out. Just because there was one reference/easter egg for M21, doesn't make it canon, especially since Momiji's appearance in the Kyoto case in the manga (1000-1005) contradicts the events of M21.so the events of the 21st movie is canon, considering they just mentioned karuta