I don't know the 777 but i know a bit about the FBW system as a similar system from the same company is used on the E-jets I teach. In our FBW we have AOA limiting which means if you pull back to the stop you will be limited to MAX AOA for the current config. i.e. Cl max. If the 777 is the same and they pulled back to the shaker after the initial power loss the aircraft would be at Cl max. If they then retracted flap the FBW system would pitch the aircraft to maintain the max AOA for the new configuration. While I am aware that Cl max is not best L/D I would hazard a guess that thats what they flew.
It's all theory but the next time I have 5 minutes spare in the sim I'm going to see what effect retracting flap when AOA limiting is active has.