We actually talked through that. She said if the engine ran continuously as the coolant level became low with a leak in the radiator it was possible, but if the engine was stopped overnight - no way. A continuous heat source is needed to build up enough vapor. The vapor evidently breaks down rather quickly in air and is rather hard to ignite.
BTW, read this report ~ this sounds more like what happened in the garage than anything else I’ve seen: http://www.koverholt.com/wp-content/uploads/2010/02/California-explosion-report.pdf