One interesting aspect of this incident is that Boeing have so far made no statement, except to say the fire is under investigation (as if we didn't know that!). They must already have a rough idea of what happened — even if the precise cause is not established.
Bearing in the mind the history of this type, it would be sensible
PR to at least rule out any link with the batteries. And if the fire is down to serious human error (ie no fault with the a/c) it should also be possible to compose a statement which would imply this. Saying nothing suggests to me there could be some new electrical issue here. Surely this is not about one faulty breaker?