Evening brown dog,
Hope you don't mind me jumping in as that's my FB post, I can attest to the technical veracity of the post. High angle shooting is something taught in sniper schools all around the world (having attended one myself), it's simple Pythagorean theory (I've attached a photo which will maybe explain it better) that everyone gets taught in school. Now i should say that the video is merely an example of someone shooting at a relatively high angle, he wasn't shooting at 30o nor at a target at 800m, and in his case changing from the prone to the full on standing would probably only been a 0.2 MRAD difference, but if he was only hitting the edge of a plate that can make the difference between a hit and a miss.
Regards,
Jack