Scary video in a way- there is a lot of potential for someone to try to imitate it and wind up severely injured (remember the one vid where the girl gets kicked in the face?) BUT that doesn't mean she shouldn't be posting things. It seems like the horse respects her bubble and is attentive to her signals, especially the 'game over' one.
I certainly see the appeal, as I will play with my horse in a way, eg, if he's been cooped up in a stall due to weather, I'll turn him out in the arena and 'chase' him. I carry a lunge whip to actually drive him off if he were to get too close, but he always keeps his bucking and bouncing well out of range and seems to enjoy the long distance version of tag. I play the same game with my dogs... not that that's a particularly better idea either, though it is generally safer.
I think the 'someone might imitate this and get hurt' thing is sometimes taken too far in the modern day. At some point people need to be responsible for their own and their childrens' or charges' safety. If a child isn't old enough to either know better or take responsibility, maybe they shouldn't be unsupervised with a horse.