I agree that there probably isn't much energy in a couple of oz of acetone. However, perhaps that is not the mechanism at work here.
Acetone is a solvent. In the past, solvents such as bezene were added to gasoline to increase the octane. Many of these were removed due to health/environmental concerns (e.g., benzene) or because they were found to be hard on engine parts especially the rubber hoses and seals. It is quite possible that adding acetone will increase the octane of the gasoline, which in turn, can increase the gas milage of some engines (i.e., those that can benefit from higher octane). HOWEVER, one has to wonder why it is not being used. Perhaps there are safety/health concerns or less expensive methods of boosting the octane?
Also, it may be possible that some engines benefit from a good cleaning. Again, there are other methods out there to clean one's injectors rather than experimenting with acetone.