The practical problem that the NPFA rule is there to address is a condition where clean outside air is introduced into the protected space near the detector. This could keep the detector from seeing the effect of a fire in the protected space.
In my opinion whether the inspector is justified depends on just what the rate of air flow into the room is during various operating conditions. If with some combination of closed door, window open, an operating exhaust fan, etc. leads to noticeable air movement into the room through that vent then you really should move the smoke, regardless of the letter of the code.
