I believe the requirement is that the connection to the vent pipe must be higher than the water level in the trap being vented. Since in this case the connection to the vent pipe is a couple feet below the trap, that requirement is not met by my installation, hence my installation does not qualify as vented.
I guess we've been lucky since we only rarely have noticed an unpleasant odor coming from the sink. My plan is to put in an AAV to fix it.
I am a bit puzzled, though, because it appears like the plumbing was actually designed wrong when the house was built in 1954, and everyone in the neighborhood will have the same problem. Did they not understand venting in 1954? (In principle the kitchen was inspected when it was remodeled 11 years ago, which would have been an opportunity to fix it. I know the electrical was inspected, but maybe not the plumbing? In my area they are very lax about permits and inspections.)