The Olight appears to be a 2X-CR123 light. Anecdotally, this can be a dangerous setup if one cell discharges too low before the other, into a reverse charge situation. I understand that reverse charging a CR123 is what leads to cell failure/rapid venting/explosions. It could have been a fresh battery mixed with a used battery, or poor quality cells where one is exhausted much faster. Luckier people have noticed light output dropping, noises, bulging buttons, etc. and been able to get rid of the light.
Personally, I moved to single cell lithium lights for this reason. And NiMH don't seem to have this problem with such danger. I am doubtful the OP's cited case had anything to do with leaning into the car battery. While car batteries are known to sometimes vent hydrogen, I don't see how that could set off a sealed flashlight. Whether or not an exploding flashlight could have set off an additional battery hydrogen blast would not be easy to determine, IMO.
I'm wondering if the flashlights aluminum body could have some how come into contact with the vehicles electrical system. A massive surge of 12 -14 volts with an amperage up to 800 -1000amps could certainly cause the 123 cells to explode. Even the standard 15 amp circuits could do a lot of damage. Its also possible the flashlight body somehow caused a spark that ignited hydrogen off gassing from the vehicles battery or other flammable liquids from the vehicle. The article does not state why he was looking under the hood of his vehicle so we cant say the vehicle was in working order at the time of the incident.