Judging by the pictures - that was not the main EV battery fire. The car would have started burning from down below. There would have been nothing left of the car.
It seems that the fire started in the passenger space. An electrical short could have startedit. But it is nearly impossible to determine the exact location.
Still. It is on Ford to explain their deduction. It is obvious that they've just shrugged this as 'your fault sue us if you dare'.
This might indicate that there are few of these undetermined cause fires with this model. Ford is not yet in danger of a class action lawsuit so they can afford to show the finger in these cases.

Can't think of anything that could involuntarily make a car burn (a dropped/forgotten cigarette for example shouldn't be able to burn a car). What kind of "external factor" is Ford thinking of?

I'm just doing devil's advocate, wondering if there could actually be foul play;... or if Ford is actually just as bad as we all know they are.

Can't wait for Western cars to catch up with current Chinese battery safety standards so that these "EV catches fire" thing stops being a thing (even though EVs already burn down much less than traditional cars).