Goodness me. Don't we get all PPRuNey quickly. A bit of perspective here. As you say this was training, and for all we know the instructor was indeed training his or her student about airmanship and to be cautious about rushed instrument selections. I know I've often included that in the brief.
Then consider that if the mixture guard was indeed installed the student would have had to go through a very specific sequence of actions that is completely different to pulling the carb heat, and if they're up for going down that rabbit hole it's unlikely that being drilled in instrument selection would have come into the student's head.
And then consider what you'd like a student's reaction to be to the engine coughing ? A reflexive pull of the carb heat or a leisurely discussion about which control to actuate ? And when the engine quits due to carb icing should they discuss whether the collective is an appropriate choice ? The Blackrock SAR accident is a horrifying counter example where slavish application of two crew procedures actually caused the accident.
This is training and in the process of training (even for military) people make mistakes, and sometimes (as you well know crab having found yourself on your arse in a field once before I believe) **** happens.