I’m guessing since the wort is hotter on the bottom of the kettle, rocking the chiller is more effective in moving hot wort over the coils vs. stirring, which moves wort faster on top than on the bottom.
This brings me to another question. The input flow of the chiller starts on top and moves down the coils to the bottom before coming straight up and out. If the wort is hotter on the bottom of the kettle, wouldn’t it cool faster if the cold water made contact with the hottest part of the wort first?