Really solid mechanics in my eyes. He gets a lot of extension and really employs his back side will when releasing the ball.
The only thing that looks a little constrained is his upper body honestly. Like everything is a little "bunched up." The throwing arm cocks up with the baseball right behind Kai's head. Seems like that might add a little unneeded torque to the throwing arm's movement. Not sure on this one - not a major thing but curious what others think.