I can only speculate.
On one hand, the accelerator muscles and the other front-side connective tissues need to be loaded up prior to shoulder rotation. “Loaded up” just means stretched so there is no slack. A person who has more flexibility than another will have to abduct the arm further to achieve that load than will the person with less flexibility.
Now, looseness in the shoulder joint is another thing. I suppose joint looseness could be confused with flexibility although I’d like to think that joint looseness might be accompanied by pain. But I’m not a doctor so I don’t really know for sure.
It is my opinion that any time the joints are operating at the edge of their range of motion, they are more vulnerable to injury. This is the case when the arm is abducted back and then the shoulders rotate forward. The one thing that I know to suggest paying attention to is to make sure he’s not opening up the shoulders early as that does place more stress on the arm.