A horse is 4+ times the size of a person. If a can of red bull will make me hyper(i'm 5'4"), I really doubt it would have a big effect on a horse.
Also I doubt making a horse hyper will really it perform better. If that was the case many people would be keeping their horses in a stall with high energy feeds before competitions. A horse that is crazy hyper is not careful over jumps or particularly responsive to its rider.
This girl is obviously not very bright or particularly concerned about her horses health, but she likely won because she has a talented mount, not because he had a can of red bull