Well I dunno about now, but mirrors weren't a legal requirement at the time (as with everything though, if it/they were fitted, they had to work)
It's likely that the instructor knew that the tester was quite hot on looking over your shoulder and doing regularly so might score extra brownie points. Temporarily removing the mirrors would remove all doubt.
...and it's maybe not so dumb. Relying solely on mirrors full of blind spots is dumb.
As a matter of interest, do you guys get taught to look over your shoulder before setting off in a car or before you open the car door?