They're just the only group who registered in their division, like Burroughs was at the Bonita Vista competition. There are so few Tier I groups in California that this ends up happening a lot. There are two other divisions here that only have one choir as well. They aren't competing against anyone but they aren't really in exhibition either... it's just a lonely division with nobody else in it to compete against. Yes, that does seem ridiculous, no you aren't crazy for having the question, and yes the separation of Tier I and Tier II in California was a terrible idea from the beginning.
I could be wrong, but didn't SoCal used to have only 1 Advanced Mixed division? I thought back in 2016 (I was in 7th grade) that all the usual tier 2 groups competed with all the tier 1 groups (Carlsbad, Hart, Bonita, Burroughs, Los Al, Brea, etc.)