I don't agree with him on mount, but I do have a multiple of thoughts on the back positions.
Back sitting is awful. Essentially, it's either your opponent mistimes a denial or allows you to move. Because if they don't want you to move, you're not going to move. There's only 1 way you can go if you want to advance, and if they don't move first, there's no chance you're getting a full stamina RNC. I think there should be more options to move to different positions because if I get denied once in back sitting DOM, I'm just going to stand up because it's a waste of stamina.
Back mount is good, but it suffers from a couple of problems. First of all, the roll from back side SUB to back sitting SUB is wayyy too easy. It's the SUB's choice if they want to give up back mount, because if they don't they have like a 2-3 second window to roll to back sitting SUB and then we go back to the first point of how bad that position is. Secondly, again, there's only 1 way to go if you really want to advance position. If your opponent just camps the up denial every time you move, you're not going to flatten them out and if you go back to back side you're just helping them. As well as that, the regular strikes from back mount DOM do basically nothing and are a free arm trap for a free getup.