Steam is an energy source when used to power a vehicle.
No, he's right, it's a transport medium, whatever is producing the steam is the energy source, the steam doesn't produce any energy, it transports it, and does so in a very lossy fashion. Heat energy is used to boil the water, which produces the steam, which spins a turbine or expands a cylinder, and the steam simply condenses back into water after being used for performing that work. This is the same with the medium that powers a turbine at a steam power plant, the input can be coal, oil, gas or fission and 30-40% of the energy used to produce that steam, makes its way out the other end of the generator as functional electrical energy, the rest is released as waste heat.
Direct use of fossil fuels was clearly less complex than using steam, which is why that transition occurred. Same with going from coal fired boilers in locomotives to diesel-electric, which is also less lossy/more efficient. Gas turbines have some advantages over steam turbines in power plants, but the most efficient are combined cycle that leverage both direct combustion and steam heat recovery to drive turbines, which minimizes waste heat loss, which, like with an ICE, is where the majority of the energy ends up going, out as waste heat.
Jay Leno also has a turbine-powered car, I believe it was made by Chrysler.
Petrol and diesel fired engines became dominant because they were more convenient than their battery-operated siblings at the time and had a portable fuel source, an issue that is still present today. That's not a dig at EV's, it's just the reality of the density of hydrocarbons as storage medium, which significantly exceeds that of any battery chemistry. The disadvantage is of course that extracting power from that medium is a consumption-based model that always requires more of the medium, unlike rechargeable storage, which, while slower, is capable of being refreshed with power produced via means other than combustion (nuclear for example).
