I am no expert, but I don't see any obvious fire damage. From the crash pictures I have seen: that plane looks like it was twisted and shredded mid-flight. Strange indeed...
Since there is an active (criminal negligence?) investigation of this airline in Russia any statement that they make may well be influenced by that
It could be a simple explanation of metal fatigue or it could be sabotage ...
https://en.wikipedia.org/wiki/China_...#Metal_fatigue that does raise suspicions