He would have detected the fire before ATC and ACARS was lost and would not have flown 8 hours after.
Prove it!
If there was a fire, we don't know where it was and how it developed.
Aviate, Navigate, Communicate
Well, if you cannot get past Navigate before whatever event it is kills you, even if your comms work, you cannot communicate. Helios did fly and none of the pilots did communicate.
Fact is, we don't know ANYTHING about what happened on board. And unless you can absoluely exclude a possibility, it is valid.