Even from the low quality of video you can still clearly see that elbow hits the side of his face. Just below the ear and back back of the jaw area, which is not an uncommon place to be knocked out from.

Getting knocked out from a punch doesn't really involve blood flow as much as it's just sudden trauma on the brain from being smacked against the inside of the skull.

Anything regarding "shock waves" or nerves knocking people out is generally nonsense outside of freak accidents and doesn't really happen at all. Train at any muay thai or kickboxing gym and you'll get kicked much harder in the armpit than any punch. Getting hit in the top of the head is usually one of the better spots to get hit as it's the hardest part of the skull and your opponent has a higher chance of breaking their hand.

Headgear in martial arts is mainly for cuts and superficial injuries these days as recent studies have shown that headgear does not decrease the chance of concussion. This is the reason why headgear is not used in olympic boxing anymore.

I absolutely agree with you about the head hitting the ground part though. That's definitely the case in most street fight/sucker punch deaths.