I think we maybe need Gengis to confirm the certification standards.
But all GA types should be able to do it.
The orignial OP's instructor isnt wrong in what they say. And personally as long as the person is back at flight idle or idle power before lifting the noise through the horizon to flare I won't say anything. It does require more skill than keeping the power on and if the student is being forced to carry out 3 deg approaches it may make it nearly impossible. Which is another point instructors forcing students to fly C150's in the same style as a Jet with no doudt adding 5knts on to the approach speed due to instructor incompetence, 10 on for gusts and another 5 knts on for mother. Stick some power on in the flare and you have a whole heap of energy to get rid of that the performance never catered for.