Well, I’ve seen a 747 land in near typhoon conditions, and the Captain was nowhere near that rough, so, I reckon that part of the reason that this one is being thrown around is - his own rough control inputs!
The airplane is stable, but with ailerons that are bigger than the wings on many airplanes, it is responsive to control inputs.
Big, violent, rough inputs like those in the video lead to big, rough movements of the airplane, instantly translated from the controls, through the large control surfaces (aileron, elevator, spoiler, likely rudder, as well) on the 747.
The seats are locked down - but the cushion itself allows some movement - and there is the equal/opposite reaction to his violent pushing, pulling, sawing Yoke motion.