Not to mention that the design constraints of a reaction wheel and a drive wheel are almost polar opposites: a reaction wheel needs to have the largest possible

Not to mention that the design constraints of a reaction wheel and a drive wheel are almost polar opposites: a reaction wheel needs to have the largest possible moment of inertia, to maximize the change in angular momentum for a given mass and torque; meanwhile, a drive wheel should have the smallest possible moment of inertia, to maximize the change in angular speed for a given mass and torque, particularly since you typically want to minimize unsprung mass (in fairness, the demoed robot didn't have any suspension to worry about).