I'm not sure I agree with that 100%. I think every shooter has an optimal range. When I was playing (back in the day), it was easier for me to hit a 3 pointer from outside the top of the key than something around 15 feet. Because I could use my whole range of motion and not have to worry about holding back on the strength of the shot. Imo, the midrange shot is one of the hardest in basketball.
I think Green might be too consistent with his mechanics which means he can't alter his shot in the least and be able to hit. As I mentioned, he rarely seems to hit a shot moving left or right or with kind of hitch if there's a defender coming at him. But I do think this has to do with his form and his release. He has a wide ranger of motion which imo, means he has to get a lot more right than someone that shoots a more compact shot. If you watch, Green jumps fairly high and releases the ball high over his head while Curry lets it go lower and from the side. And on an open shot, doesn't jump high or extend his arms much at all.