The binding won't open while your weight is on the boot, ie you have to lift the lever and unweight/lift your boot at the same time to get out.
When on a heelside turn the weight of your heel on the baseplate keeps the binding closed. On the toesdie, the weight on the ball of your foot on the baseplate is enough to keep the binding closed on a hard turn. The lock is really needed for any type of air or when your boot is unweighted.
We have ridden the binding without a lock purely as a test. It worked fine for normal cruising, but definitely would not recommend trying that.
If a branch or other was to flick the handle it will flick back down & lock as soon as the branch was gone and you probably wouldn't know. If you collided with a bird while in the air, you could possibly have a problem.....
The aim in the video was to show the ease of exit, but probably could have shown that better & provided some more explanation.
Hiback levers have had a tendency to unlock in the past, especially when sliding on your back. We've place it in a position where it seems least likely to be hit by something. The front binding would be more likely than the rear if there was an issue. More fine tuning to do, but so far no issues in this regard..