HP:-
I'm not so sure that saying is used to help you decide which boot to move, rather than help you diagnose which engine has failed from the resultant rudder deflection!
Agreed, and they had already identified the failed engine (#1) but an incorrect left rudder input was subsequently applied. The Report considers that P1 was suffering a sensory overload. If he had simply remembered that simple mantra he would have realised that his left foot should have been the 'dead' one, ie applying no rudder, and fed in right rudder instead. The sideslip that killed them could then have been avoided. Given how much else was going wrong it is by no means certain it would have saved them but it would have been a start, at least...