I don't think it makes much if any difference what the materials are, once a fire has taken hold inside the cabin no amount of external water/foam can have any effect whatsoever except for boundary cooling. Unless you can achieve foam delivery inside it's just going to run off the fuselage onto the ground.
Sure, fight external fire with every effort to prevent burnthrough, but if/once that occurs you've lost the battle, and with the massive damage in the belly of this aircraft the hull's integrity may well have been compromised anyway.
I suspect we're going to be hearing a good deal about the health hazards of burned carbon fibres too, an issue that's barely been raised so far. Removal and disposal of the wreckage will be a difficult business, especially if there is any wind at all.