I would treat all reports of that "heat flash" with caution. It was reported, then denied, then reported to be something else not the aircraft. Until it is confirmed by an official US/RU source, I would not build any theories on it.
That being said, there seems to be no mention by anyone of the (one or two) auxiliary fuel tanks in the rear cargo hold, which is essentially a purpose-built ULD and a standard fixture of 321s on longer routes. If the rear fuselage disintegrates, those would be dislodged and ruptured, liberating enough fuel to produce a fireball if ignited.