So the legalities that I see are:
L4 STUNTS J2
L4 GENERAL TUMBLING B
So in a downward inversion, flyers must be assisted by 3 catchers, 2 of which are positioned at the head and neck. In Clarification 2 it states that the catchers must make contact with the waist to shoulder region to protect the head and neck. There is no one on the waist to shoulder region of those flyers.
Under general tumbling, you are not allowed to tumble over another individual (torso over torso).
So as far as the stunt in that video, it would be illegal at level 4.
However, the only way to get an official ruling is to email usasf rules and submit a video. Keep in mind the rules on their website, you cannot send a video of another team, you'd have to try it yourself and submit the video.