I was just pointing out that the word spark applies both to incandescent particles and (since the 18th century) intermittent electrical arcs.
I suppose 'ember' was technically incorrect, but solid incandescent sparks can ignite material well away from the source of the sparks, while an appropriate fuel/oxidizer would need to be in the path of an electrical arc to be ignited by it directly. On a side note, the sparks from things like sparking wires are generally incandescent particles rather than electrical arcs themselves, especially at lower voltages. Anyway, the distinction is academic and says next to nothing about how effective a given spark is at starting a fire.
Most electrical fires don't feature sparks at all, except perhaps as a side effect. A poor connection, exposed wire, or a damaged wire can easily raise the resistance of a circuit (short or otherwise) to the point that it heats up enough to cause a fire. Even completely intact wiring and fixtures can be the cause of fires if there is too great a load on them and the fuse/breaker system isn't working.
Anyway, in the case of the Notre Dame fire, there is no reason to suspect it was intentional, and in absence of other compelling evidence, I'd take the official report at face value. A good arsonist can certainly make a fire look accidental, but there is usually no reason to bother, since the odds of getting caught (at least in a well trafficed venue or one without significant surveillance) are negligible and it's usually much easier to guarantee damage if one isn't trying to obscure the nature of the fire.
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