Hi Joseph,
Most of your points are correct and still stands. However, this corridor is very much used by commuters from communities in the north in Mira Mesa and Rancho Penasquitos traveling to and from Kearny Mesa which is the 2nd largest employment center in San Diego County behind Sorrento Valley and ahead of Downtown SD. 
But as for why It is also the only local non-motorway road north of Kearny Mesa that goes north-south so local traffic funnels through Kearny Villa Rd between Kearny and Mira mesas, and this is especially important during rush hour as it absorbs I-15 parallel local traffic. If there were more local roads cutting across MCAS Miramar in the parallel direction, I might agree with you. But due to it's importance in the network and controlled-access expressway nature, I was inclined to tag this as trunk. This is very much not a typical primary road as it is performing above the use of primary road.
As per https://wiki.openstreetmap.org/wiki/California/2022_Highway_Classification_Guidelines, this urban expressway  can be tagged as trunk.

Personal disclosure: I was a commuter on this road, as well as my coworkers. So I recognize that I may be biased based on personal experience or lack-of personal experience commuting on I-15. Neither trunk, primary, or motorway tags do this road justice; but I believe trunk is probably the best tag case.