You made some good points, I am a trainer and agree 100% about the issue with the rotator cuff and the problems he's setting himself up for with his neck.
I cringe when I see people doing stuff like that.
Another pet hate is when people give the advice to keep the shoulders back like that when soemone's doing a military press, either with a bar or dumbbells ect. It's the same thing, it puts the shoulders in a very precarious unnatural position and wreaks havoc on the rotator cuffs.
I saw someone giving that advice the other day to two girls. I literally stared at him and wanted to say something, but I just couldn't. There are so many people at the gym that give such bad advice to their friends, because someone told them something wrong and they didn't do research to learn they were told something wrong.