I think there are more than three possibilities. One possibility is to exert the force necessary to keep the person upright without moving forward or bckward. How would the device know when and how to do this -- by monitoring the hand grip and foot pressure. It would be possible to tell the difference between the intention to move and being pushed off balance. This thing was in development for years, and the creators are not stupid.
How would the device do this? The only ways the device has to 'exert the force necessary to keep the person upright' are to either (1) have the motors apply torque to the frame try to keep the thing upright; since the motors connect the frame to the wheels, this torque applied to the wheels will cause them to turn if the vehicle is on level ground; (2) move the machine (by applying torque as above) move the wheels such that they are again under effective center of gravity (adjusting the COJ for outside force vectors such as wind).
That the machine can quickly and effectively provide the required movements to stay upright in an 'aparently' stable fashion is very impressive. But staying upright requires that it be able to move.
The circumstance is somewhat akin to trying to balance a broom (bristle end up) on your hand, though the torque behaviors are a bit different. The key observation is that trying to balance the broom on your hand requires that you move your hand. If you're skillful you won't have to move your hand as much as if you're not, but you'll still have to move it. If you stop moving your hand the broom will almost certainly fall.