Retarding the thrust levers before selecting the 'hard alternate' mode is necessary because there is no thrust limit protection in the alternate modes (in case of the BA038 situation the thrust levers would have been fire-walled). If the crew had decided to select the 'hard alternate' mode (regardless of an EEC fault or not) they would have retarded the thrust levers before doing so.
If you're out of thrust, retarding is
not necessary.
What if they had firewalled with such a switching? - It would have saved their day, wouldn't it?
This is some silly behind the desk comment. If you are in a emergency situation without thrust, you really don't care about technicalities, formalities, bulletins, sops or similar: Anything that gives you a notch more is very welcome....