I remain unconvinced that a passenger, already submerged in icy water, had the clarity of thought to say "no point in deploying the rebreather because I forgot to take a breath prior to being submerged", or, following implementation of this AAIB recommendation, would now say "oh dear, I seem to be underwater with empty lungs (having miraculously resisted the gasp reflex), I'd better deploy my rebreather and use the air in its cylinder".
HC, while my gut instinct is to agree with you, I learned a few things about "negative training" when I was in the service (particularly about survival gear) and feel less critical of the point AAIB is making.
There are a variety of tools/types available. It seems a valid approach to refresh one's memory before a transit on what tool is being provided, and how it works. I agree that initial training, and recurrent, ought to address this, but a little refreshment never hurts for life critical skills.
When I was instructing in fixed wing, in a parachute equipped aircraft, my students and I on every flight briefed and went through the procedures, all steps, for bail out. (A very rare event). Why? To refresh the mind on a particularly "you only get one shot at this" event, however low in probability it might be. I got a lot of positivie feedback on that approach.
When we flew from ships, a number of tail rotor malfunctions were addressed in the brief, top to bottom, since those malfunctions were of critical nature to get right if one ever cropped up in flight (rare, but not zero).
For a passenger flying over the North Sea, refreshing just what that survival tool does (and these guys don't transit every day, right? Once a week? One a fortnight? ) puts them into an alert mind set, which, God forbid it all goes pearshaped, may make the difference between successful or failed emergency egress. You only get one chance at it.