I wonder what really happened that he came up with that story to tell the cops? I'm not sure I'm going to believe this story until we have Adam Savage show it's possible.
Adam Savage already blew stuff up with spray sunscreen (for the same reason—propane as a propellant). The hard part is getting the stoichiometry right so it explodes instead of just burning.
If you contantly have source of ignition and add fuel, there will be a time when mix is just above sustaining near ignition source and more rich elsewhere. It doesn't take that much pressure to blow out windows in a car, it doesn't need to be properly mixed at right ratio in whole volume of car. It doesn't even need to be explosion, but very fast burning will increase pressure enough to blow out windows.
Vaporized fuel + air explosions is essentially what occurs in the cylinders of a gasoline engine of an automobile; there's nothing mysterious about this.