All the imagery I have seen from the avionics bay shows little (if anything) flammable. All the electronic assemblies are in sealed aluminium boxes, neatly stacked on racks. The wiring will be the usual a/c grade used by Airbus, probably their "TKT" which has been used since 2006, you won't be getting that to burn very easily in a normal environent. There's a whole load of insulation material too, again, good luck on getting that to burn.
The sequence of events on ACARS looks highly similar to the MS677 incident ... from the window heat sensors, it even started in a similar place. Looking back and with the Egyptian habit of finding a "politically expedient" answer, it is at least possible the MS677 incident was not "just" an electrical problem/oxygen leak, but something else. Maybe this was their second run at it, and this time it went off in flight, not on the ground.
The first officer said there was a "bang" from the right side of his seat and he saw a 10cm "crack" appear in the side-wall adjacent to the oxygen mask. The cockpit-voice recorder captured a "pop" followed by a hissing noise, similar to the escape of pressurised gas, says the inquiry.
"I unfastened the seat-belt immediately and stood up very quickly," the first officer told investigators. "At the same time the captain left his seat quickly. The smoke and fire were spreading very quickly. After that, the captain ordered me to get out of the cockpit."
The captain attempted to extinguish the fire but said: "The fire bottle was completely depleted without any influence on the fire intensity."
OK, so different a/c type ... but, if it had been some sort of "device" rather than a fault, do you have confidence the Egyptian authorities would have fessed up?