I notice that on the pprune threads about developing stories, there is quite a bit of grasping at straws based on tidbits from the press, balanced from time to time by reminders to wait until more is known.
Of course, it's possible that the ELT spontaneously erupted into flames, presumably from its battery. I wonder, is there any precedent for that?
But it's also possible that folks investigating the 787 found the ELT with a burned-up battery, and asked Honeywell (quite appropriately) for assistance. This is not enough to say that investigators have concluded, or even deemed it likely, that the ORIGIN of the fire was in the ELT.
If I understand correctly, Li batteries can "flare up" when put in a fire -- in other words, a fire originating elsewhere could perhaps ignite the ELT battery.
It's early days yet, and we have so few facts.