Not being familiar with a 777, what would have caused that intense fire? Didn't look like the fuel tanks ruptured and started the fire. Does the 777 carry LOX near the main cabin area or is there just that much flammable material in the cabin?
Oxygen lines to the PSUs
Even if this Korean 777 was fitted with a gaseous oxygen system and not a chemical-generated oxygen system, the lines to the PSUs should not have oxygen in them without the system being activated.
Perhaps an unfortunate combination of power and shorted wiring would do this.