Most slamming the hips closed is to prepare for fielding the ball. Once it is released you can do what ever you want and it has no impact on the pitch.
The discussion of "Slamming Hips Closed at Release" spans a broad range. Some D1 pitchers leave their back hip way behind and never rotate through the pitch. Then there are Step Pitchers who tend to keep their hips square to the target most of the motion. Then there are thousands of variations in between. I agree that the first stride requires the body to open their body to the target. I agree that Mowatt and Ueno both turn their hips from open to have their hips at 45% at the point of release. They also continue the hip turn after the release. I don't consider this "Slamming the Hips" but I do consider it a natural motion to follow the shoulder turn and release. It does also aid in getting the pitcher in balance to continue the play.