Interesting take. Allow me to propose a different story, along the lines of what Bobrow pointed out but didn't quite get to the bottom of. And you can find my opinion on what backward tosses do to serve quality within it.
Miyu Nagasaki has a very illegal backwards toss on a variety of serve actions. It was probably noted by the CNT as something to watch out for. Initially, Miu wasn't largely doing it so it was kept away. Maybe she had been tipped off that the CNT was going to challenge it.
Now she was down 0-3 in games and 0-3 in points, getting no easy points on serve and unable to hang with the best player in the world. Her last chance was to introduce her usual weapon, the backward toss that enables hiding and tossing the ball into the racket which gives the ball heavier spin on the contact action. So her coach tells her to try it and see whether it gets called by anyone. And during the timeout, SYS is reminded that the serve has been clean so far but this is probably her opponent's last chance to make it a match. So when they come out, rather than battle through hidden serves or tosses that are hard to read, SYS just nips it in the bud to keep her victory uncomplicated. Because all it takes is momentum for an easy victory to become complicated.