You are probably installing it too close( <2x diameters) and downstream of the throat. The inspector is right, the eddy currents may miss the sensor as it swirls past. It is just good practice and it is a shame he didn't share the explanation OR was hoping you would disagree so He(?) could show you how smart he was when you had the audacity to question him/her
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!