The anti siphon flush valve is not intended as a vent. But you are correct that it does have a direct vent to atmosphere under that cap and any positive pressure in the black tank could push air back up thru it. And that atmosphere is inside the RV.
If the vent opened, and all the water drained out of the pipe into the black tank, and the tank water level was below the spray head, then any positive pressure on the black tank could push odors back up that pipe to that vent. The tank roof vent should prevent positive pressure. But positive pressure could result from flushing with a blocked or partially blocked roof vent, or strong wind blowing just right to push air down the roof vent, or …..
Have heard people mention unexplained sewer odors but never heard of anyone actually tracing it to that anti siphon valve. Certainly possible.