Everything comes from the core. Throwing, pitching, swinging a golf club, swinging a bat, all the same principles. All the energy comes from the core and is transferred through your arms and hands to the ball/bat/club. If you start with your arms/hands, no energy gets transferred and you're a lot more likely to injure your arm because it's doing all the work.
My son plays college baseball and it's just a massive difference between how he learned to hit and how us old guys were all taught in the '70s and '80s. I get in the cage on a lark and I can feel I'm all arms and hands. I can still hit at my advanced age, but not with anywhere near the power he has, and it's not just because he's a hell of a lot stronger than me.
Josh has the physical gifts to throw and play at a very high level despite mechanical flaws. Now that he's fixed those... that's why he's a unicorn. Most guys with those physical gifts never feel the need to correct their flaws.