...but he IS teaching the wrong thing, which IMHO is not acceptable.
Wrong maybe, but not safety relevant, therefore unimportant to me! How many instructors (even textbooks and POHs!) confuse "mass" and "weight"? Physically as different as speed and accelleration. Yet again: Totally unimportant.
There has been a thread recently about some guy demonstrating single-engine stalls in a twin below Vmca. That would be something to talk about with the instructor concerned. Urgently. Even publicly, if he won't listen privately. But "knots per hour"? I couldn't care less.