That drill does not take into account the fact that a prevalent challenge in “staying closed” with the shoulders is that the hips are opening when they should, just before landing, but the shoulders shouldn’t. Competing things happening in the brain. At the same time that this struggle is going on in your brain, you are also trying to get the front arm doing one thing, keeping the glove from turning over, and the throwing arm is doing it’s thing in an entirely different way. Oh, don’t forget to think about how you want the ball to face, or where you want your fingers, either on top or on the side but, wait, when should I do this and should I use the M, aaaaaaahhhhh!!! Toooo much for the brain to comprehend.
Now I know that all of the drills proponents like to break things down into component “chunks” in order to quite all of that noise. I believe that this is a big challenge for those types of drills, including the one you’re proposing. That “noise” all happens AT THE SAME TIME when you pitch in a game situation. So, one really needs to ensure that training includes that, at some point. I don’t believe your drill is going to be terribly productive and transfer into a situation where you add the noise.
My suggestion is to work backwards in the pitching sequence, using a reverse progression approach. Mills recognizes its benefits, calling it “pitching backwards” and Nyman has written about it several times. It can be very effective in getting the “feel” for how all of these competing motions in different areas of the body, at different times, actually go together in a unified whole.
I think one needs to practice having the arms do their thing while the hips rotate into landing. Let the brain get used to those disparate elements working together rather than against each other. While the ball is being lifted upward toward the high zone, the bottom half is rotating into landing. That needs practice.