The instructor is apparently a 747 captain or FO, so experience should not be a question (though I don't know what his experience of flying relatively low in small aircraft is).
So am I...but looking around at the others I work with on a daily basis, I'd recommend you don't place a lot of faith in an instructor simply because he flies a 747. A 747 isn't a Seneca, doesn't have piston engines, has considerably greater capabilities in ice, has substantially higher capability following an engine or system failure, and unlike the Seneca, lands a whole lot higher in the air. Trying to go between the two is a good way to get hurt if one isn't fully prepared. I know a number of 747 pilots I'd happily fly with in a big airplane, but wouldn't trust to fly my dog in a light airplane.
No - I would not castigate this instructor as some have, but if he was picking up clear ice from supercooled rain and the 0 degree isotherm was below his MSA, then I would heartily agree.
Whether the aircraft was in supercooled rain isn't a necessary factor to consider the original posters comments. The aircraft was cited as being flown in moderate ice. The next step past moderate is severe, and by definition severe means the ice is beyond the capabilities of the airplane to remove or control it.
The example set is a poor one. Anti-ice in a light twin exists for one reason only, and that reason is not to allow it to fly in the ice. It's to get it out of the ice.
I once sat through a ground school for the Twin Commander, in which the chief pilot asked who knew how much ice the TC could handle. None of us had an answer, so he announced with some certainty that the airplane could handle all the ice one could throw at it, and would lose only fifteen knots.
A few days later we were returning from a desert location to a mountain location, and were climbing IFR and IMC below the MEA. We were in an area of rapidly rising terrain, which was forcing a considerable amount of moist air upward. We hit the icing about the same time we were cleared to the MEA. In one minute over an inch of ice built, and we lost 50 knots. The airplane passed through blue line, then approached redline, and was descending with full power, with full heat and boots, and no sign of improvement.
ATC finally kicked us loose because we couldn't make the MEA at that point, were descending, and out of radar contact. We continued to descent until we made ground contact, close to dark, and were able to reduce the descent rate coincident with terrain, until we reached a rural airport. While that was going on, the sound of ice coming off the props and hitting the cutline behind us sounded like 12 gauge shotguns going off continuously.
The next morning we were called into the owner's office and asked what the holes were doing in the side of his airplane. We went out to the fuel pumps where the airplane was chocked, and found what we hadn't seen in the dark; considerable damage done to both sides of the airplane.
A few days later, I heard the same chief pilot tell someone that sure enough, the airplane could take all the ice one could throw at it...and lose just 15 knots.
Just not true. Airplanes ahead of us on the same routing didn't get the ice, and behind us. We pased through an area of significant ice build-up due to orographic lifting. Ice can build quickly beyond the capability to control, and should be given a great deal of respect. It doesn't take a lot of ice before you're flying an entirely different airplane...different performance, different characteristics, different capabilities. It can become adverse.
Ice shouldn't be taken for granted. Anti-ice systems shouldn't be taken for granted. Build up some ice, blow one boot and have the other fail, and now you have an assymetrical airfoil with two different lift capabilities and handling...on different sides of the airplane...and you're now a test pilot.
Especially in a training environment, playing with ice is a very foolish thing to do, or to show to a student. Particularly in a light twin.
Two years ago I had occasion to take an assigment in a light turobjet airpalne that required carrying about 3" of ice all the time, and maneuvering it to the lower performance limits (shaker and buffet) regularly. Several different times I found unforecast, unadvertised adverse handling characteristics in the airplane. This is not desirable. This is not something you want in training, nor in an airplane with limited performance, nor an example or habit to be developed.
The instructor who perpetuates this hazard does no one any favors.