It looks like the game against Chaminade is exempt from the 31 game maximum due to an NCAA rule about playing games in Hawaii.
Here is a link giving information about that.
The opener against Chaminade is in part made possible because of an NCAA rule regarding the University of Hawaii, which sometimes has trouble scheduling home games. NCAA rules permit a team playing in the Maui Inviational to play a game at Hawaii as essentially a "free" game that does not count against the maximum; that free game is why the Tar Heels can afford to play Division II Chaminade in the Maui opener.
And here is a link with the rule, although not from an NCAA source
220.127.116.11 Once-in-Four-Years Exemption – Contests in Hawaii, Alaska, Puerto Rico
or Canada. Once every four years, an institution may exempt from its maximum contest limits a
maximum of four contests during a single trip to Hawaii, Alaska, Puerto Rico or Canada, either
against or under the sponsorship of an active member institution located in Hawaii, Alaska,
Puerto Rico or Canada, by a member institution located outside the area in question.