So its not a surprise.... Engineers have been designing systems, with the assumption that batteries are not going to fail. With the technology that we had... that was fine. Batteries didn't fail, so having a system that in this particular scenario, required the APU to run, to operate the vents, to control a fire... was fine.... because the battery wouldn't set on fire and go off line in the first place...
My point is that it is clearly not enough to simply insulate the batteries a little more and vent any fire out of the aircraft.... Thought has to be given to the implications of failure of batteries, that appears to have been a scenario not worth bothering about previously.