The car in question belongs to Robert Carlson, who is also a Tesla investor. In describing the freak nature of the incident, Carlson makes the point that even though the fire looks bad in the video below, now that a Model S has caught fire in the wild, the fact that the battery and car did not totally immolate proves that, "this 'doomsday' event has now been tested, and the design and engineering works." CEO Elon Musk further explained that the piece of metal that probably hit the Model S fell off a semi trailer and would have needed to be pushed up off the road in a lever-like action with, "a peak force on the order of 25 tons," since, "only a force of this magnitude would be strong enough to punch a 3-inch diameter hole through the quarter-inch armor plate protecting the base of the vehicle."
"This 'doomsday' event has now been tested, and the design and engineering works" - Robert Carlson, burnt Model S owner.
Given the thin metal that encases most gas tanks, Musk writes, gas-powered cars are "vulnerable to destruction of the fuel supply lines or fuel tank, which causes a pool of gasoline to form and often burn the entire car to the ground." Since the Tesla battery pack has firewalls inside it (which worked well in this case to keep the flames from engulfing the car), "the effective combustion potential [of the Model S] is only about 1% that of the fuel in a comparable gasoline sedan." Musk also describes why the fire department had trouble putting the fire out at first. You can read the full argument on Tesla's site, including why Musk maintains "you are 5 times more likely to experience a fire in a conventional gasoline car than a Tesla!"
"You are 5 times more likely to experience a fire in a conventional gasoline car than a Tesla!" - Elon Musk