As long as the legs are providing 100% of the movement, what's the difference? If the other parts of the bot touching the floor are not powered or at least don't contribute to the bot's movement, then they don't take away from it being a walker with 2DOF.

Saying that the legs can be the only thing touching the floor is too restrictive; what if the bot was knocked on it's side - would it suddenly stop being a walker?