The video does not show the few seconds before the collision and I wonder how the left wing of the jumbo managed to miss the smaller aircraft's rudder.
The flight crew of the jumbo probably should have seen the smaller aircraft, since it was apparently in their field of forward vision. The flight crew of the smaller aircraft did not likely see the jumbo, which struck to their rear.
Whoever managed to take the video presumably saw what was about to happen; the ground controllers stationed in the tower must somehow have missed it and apparently were unaware of it until called by the flight crew of the smaller aircraft.
"...I wonder how the left wing of the jumbo managed to miss the smaller aircraft's rudder."
Well, that is really simple....it DIDN'T.
The outer left wing of the A380 hit the rudder/vert stab of the RJ.
"The flight crew of the jumbo probably should have seen the smaller aircraft..."
He undoubtedly did see him, but the RJ was probably moving into the alley as the A380 cockpit went by, then stopped unexpectedly short.
I assume both pilots will get dinged by NTSB, the RJ for not clearing the taxiway as instructed, and the A380 for not keeping an adequate distance from the RJ (although that is damned hard to judge, exacerbated by night, lights, rain etc).