With a fuselage rupture of any sort is it at all creditable that there is any chance of fuel catching fire or exploding at that speed and that altitude. I would suggest not.
It is true that fuel is so difficult to ignite at altitude. Even in engines. But it is not impossible. You need enough initial energy(heat) and reduced airflow to sustain flame. Though I have read that chances to ignite jetfuel reduce exponentially as temperatres and pressures drop. Thats a whole science.
Anyway, rear passengers and seats got scorched somehow. And there was a heavy fire on the plane before it impacted terrain.
I suspect that fuel fire was secondary event, caused by series of events before primary destructive event.
I wonder if this particular A321 had additional central fuel tank or no? If yes, it is located aft of wings inside fuselage.
Standard range with winglets for this plane type is 5900km. Sankt-Petersburg - Sharm el Shaikh distance around 4000km. Anybody knows?