I believe the target needs to be staggered for you to get the damage bonus, as written in the description. The enemy in the video isn’t shown being staggered when you hit it.
Yeah its always been weird. If you can’t knock them down you can’t build use the bonus, despite stacks accumulating even on non staggers like whacking enemies in those animations. But with 8 stacks and 10% damage per they would probably nerf it if it were to work as you’d think.