Lancair70, that part of fuselage is from just under the second door. The door that is located in front of wings.
there is actually similar pieces from opposite sides of these doors that have similar fire and heat damage.
http://www.keri.ee/crash/siinai3.jpg
if you look at the nose, then you can see that underside of nose is also covered with soot.
Russian medics said that main reason of occupants death was fire and blunt trauma.
Now back to speculation,
Regarding all this, it seems like after plane has lost its fuselage behind wings, its broken fuel tanks fuelled a large fireball around the plane. While remaining plane was falling backside down. It takes several minutes to fall and burn. If you look at the flames direction then it coincides with the probable falling attitude. The closer to the wings, the more heat damage there is.
Also there was a picture of a separated engine fan having similar sooting and heat damage. Thus, wings and engines must have been inside the fireball of falling plane.