Not sure this has been the official "explanation" this time, but looking at it from a technical side, there isn't normally anything in a transformer flammable enough to be ignited by a cigarette, even if you could drop it directly into the cooling oil (which you can't: they are normally sealed). My understanding is that you need a sustained arc over several minutes of "normal" electric current, or several lightning strikes to heat up the oil enough to catch fire. That requires some major fault. I guess a suitable type of warhead could cause it eventually, but not immediately.