Why does this guy talk about external rotation when covering the topic of rear delt activation (at about 6:00)? I don't see how external rotation helps me extend my shoulders further backwards (I tried it both with and without external rotation). Plus, it's obvious rear deltas don't rotate my hands (forearm muscles do)
When he says "external rotation", he demonstrates supination of the forearm (I.e. turning the hand so that the palm is up) without any shoulder movement. So it seems that he's talking about forearm supination, which is external rotation of the forearm from the radioulnar joint, rather than shoulder external rotation, which is rotation of the humerus from the glenohumeral joint. It's extremely doubtful that this makes any difference to either shoulder extension range of motion or contribution of the rear deltoid.
However, the earlier statement about getting into shoulder extension rather than a reverse fly motion (which would be shoulder transverse abduction) is correct and relevant: the difference between the two positions is that in shoulder transverse extension, the shoulder is internally rotated, while in transverse abduction it is externally rotated. The internally rotated position is probably better for rear delt isolation, because the side delts can contribute to the lift in transverse abduction, but not in transverse extension, so you are relying 100% on the rear delts in transverse extension.