You do good research. You bring one question to my mind. Thermal expansion. How long prior was the car driven and how long would it take for the pressure to subside? I have not read near enough on this story but if she was only home for an hour or two would that have caused the pressure to remain much higher than you anticipate with a ‘cold’ start?
The timing would depend on how cold it was in the garage, but it wouldn't take very long under most circumstances. NOAA says that the low on January 10th was 19 degrees at 6:41 AM. This garage does not appear to be heated so it was (guessing) probably about 15-20 degrees warmer than that.
Think about it this way, everyone knows not to remove their radiator cap while the engine is hot. Reason: The coolant is under pressure from thermal expansion. I think that waiting an hour it would certainly have subsided for the most part. Remember, it not only has to be under pressure, it has to be under enough pressure to atomize the leaking coolant, and maintain that atomization long enough for a flammable cloud of coolant to reach the ignition source.
I smell BS