People fall for these things because they have read from authoritative sources that their engine is about 30% efficient. Then when the snake-oil salesman tells them that they are only burning 30% of their fuel, they believe it.

In reality, you burn nearly all of the fuel in the engine. The <1% that you do not burn is not enough to affect mileage noticeably.

Of course, 70% of the energy is being released as heat. If there comes about a technology which does improve mileage, you can be sure that it is because less heat is being wasted. They will indeed dwell on this fact.