It is only pretty because it is in in slo-mo. It was an arm punch. Look at the positioning of his body and feet. The body punch was far superior.
It was the body punch prior to the left hook that left himself open to that left hook. Anyone can punch a hook at that moment and makes it look good.
You do bring up a good point. That depends what kind of hook you are trying to execute. On a counter hook, weight should be on the back foot as front weight left hook leaves your momentum forward. As you can see he ends up tipping over on a whiffed double hook. You could also see his weight transfer a little wonky. His front foot is stiff which is not bad but a bit wonky in this instance because his back foot is stepping back and lifted.
Please don't overestimate. He could have thrown any punch at the wobbled and hurt man, it would be just as spectacular.