OK, let's clarify 'open' and 'closed'.
To me, 'open' is facing 3B, and 'closed' is facing the batter. It seems that you're saying the opposite. Let's clear that up first.
Right. I think the common terminology as that the hips start in the closed position facing 3rd base for a right-handed pitcher and then open toward the batter. If I've got that backwards than I've had a lot of embarrassing arguments over the years with people who probably actually agree with me.
It looks like you mean the same thing but are using different definitions of closed and open which might be some of the reason for the confusion.
When I watch that video I see no separation between lower and upper body. His left arm is bringing his back hip through - they come to the plate together. What I think I hear you saying is that his hips should be rotated towards the plate (vs open or closed) when he begins his upper body delivery.
When I see that video it reminds me a lot of my 2016 RHP last summer and BFS's description is spot on. He had two coaches at the time which we reduced down to one after the summer. The focus was on eliminating the "swing" of the front foot and separating the lower half and upper half. He did not suffer control issues but lacked velocity. By addressing the front leg swing and working on separation (what they called it instead of closed vs open) he was able to pick up 6mph on his fastball between last summer and the spring HS season (he also got much stronger). He still has a tendency to fall back on old habits though so it's a work in progress.