The same way one can walk, or run, the person using the device can adjust his or her speed to their conditioning. I don't really understand the dismissal of the idea because one could go fast with it.
Joystick controlled chairs I've seen have had the joysticks as dead-man's handles too.
I suspect that would lead to some reluctance to embrace the idea completely.
From the perspective of disabilities the demonstrator is certainly using their legs a lot, it seems the usefulness would be for lower limb amputees primarily?