I don’t have much to add except that I agree that it is not an expressway, but trunk makes sense to me. Plenty of buildings, businesses, and driveways directly adjoining it without access control or median and also an important regional road.
On the extreme end of things, it maintains this tagging even in Pacifica (below) as you encounter traffic lights and driveways to numerous businesses (and similarly in Half Moon Bay)
Less extreme, but still very not expressway to me is Davenport where there are buildings and driveways right into the highway.
And this type of thing is happening in smaller amounts all along the length.