Is this EXACTLY what he wrote? If so, he needs to re-visit his anatomy text; I'd love to see when the biceps and supraspinatus became "joints."
It's true that if you have faulty function of the scapula and/or glenohumeral joints, you'll be at risk for impingment in this position (or basically any position around 90 degrees abduction/flexion or higher). BUT, does that mean you shouldn't train your external rotators? That's just retarded.
If someone is getting impingement, I always start with the scapula and figure out what's wrong there first. I highly doubt, though, that external rotations are going to do that much damage, especially when they same trainee is probably bench pressing, overhead pressing, or even worse, upright rowing and dipping.