The big advantage of their system is the regenerative brake system. Other than that, there is little energy savings over a standard transmission. Knowing the type of route their trucks likely drive in the Detroit area (lots of stop and go) explains the 40% to 50% fuel savings. A route with more highway cruising would probably save less.
Another possibility, although not mentioned in the video, is using an electric motor when the vehicle is parked to pump the fluid back to the high pressure side.
