Triumph61, the burn/heating pattern you can see on that skin for me is consistent with fire on the inside on the ground, when thin skin heats up fastest.
I have never seen a frame/stringer reinforced (and chemically milled) skin with heat damage caused in flight, but I would assume it looks exactly the other way around, heat is best dissipated to airflow at the thin skin, because of that stringers and frames see the most severe heat and hence damage.
What we can agree for sure is heat damage from the inside, just below the floor. As the floor was destroyed at impact, it is hard to believe that it was a barrier for the fire on the ground...