I have no flying experience but if there was a fire in the cargo hold wouldn't the crew at least send a message? I thought this flight veered off the flight path before disappearing
In general, onboard fires are one of the most dangerous scenarios in-flight, as I understand it. Crews are trained extensively for it.
I think this is, in part, what the analysis in the article concludes — even a fire would have to cause a very specific, never-before-seen combination of failures to lead to the outcomes observed from the outside.