... the difference in solar heating between composite and aluminum will not be vast and given the number of these in service (over 6,000?) if there was a marginal temperature effect we would have seen it by now in something else.
But how many of the others sat baking in a dessert for 3 months?
Another difference could be what happens if a battery does go, do the aluminum instalations have a heat sink effect and or nothing flammable nearby?
It seems that something else had to have caught fire to cause the level of damage seen.
It is also possible that protective insulation (if pressent) around the ELT allowed higher temps to devlope from a battery "thermal event".