Im looking at the picture.... if the fire originated INSIDE the phone there would be a lot more damage. Look at the connector.... look at the area the wire tends to fray.... Also if the "fire" was inside, wouldn't it stand to reason that the entire opening would be charred like the one area. Not saying it didn't happen but the phone was not the problem.
The area where is frays is just melted, not black like the connector. And most likely it was electrical arcing, not a full on fire.
You can even see where the plastic shrunk away from the heat.