From the web site:
The HHOS has been tested in two different vehicles (the current prototype, which is a 1994 Ford Escort Wagon, and a 1998 Ford Ranger pickup) and fuel economy increases have ranged from 22.9% to 100% depending upon the amount of electrical energy (amps & volts) that are available for the production of Aquygen™ Gas.
Looks like the details aren't as impressive as the video.
Actually it does mention use of their system as the only fuel, but they don't seem to have reduced that to practice yet.
What it appears that they are doing is increasing the efficiency of the overall system, over pure gasoline or diesel. The energy to produce their "gas" which may be monomolecular hydrogen, not H2, clearly comes from the burning of the composite fuel. They claim a horsepower increase of 17 HP, and estimate the system to produce their "HH0" gas eats up about 4 HP of that. Thus what they must be doing is increasing the overall efficiency of converting gasoline (or JP-4 in the case of a Humvee) to mechanical horsepower. Not magic, but worthwhile.