Thinking of a pendulum reminds me of old hypnotists swinging a watch on a chain in front of your eyes to induce a trance. Putting and chipping technique is often described as being pendulum-like. Not sure what the connection is: who would think that golf could alter your mental state!
The reason for the pendulum analogy is because there is very little movement at your wrists with chip shots and putting.
Your full swing is different because you have to allow for hinging your wrists. If you think of your left arm and club, this makes the movement more like a double pendulum. That is because there are now two hinge points; one at your shoulder and one at your left wrist.
If the movement is purely pendulum-like then there shouldn’t be any independent manipulation at your left wrist. As the video describes, when your left arm and the club fall from the end of your backswing, they want to swing into a straight line at the very lowest point of the swing. That point is under your left shoulder.
When you think that your ball position for iron shots is before the club swings to this low point, it means that if you swing the club the same way the double pendulum works then you’ll strike the ball with a descending blow. Exactly what you want…
Practice the drill in the video using just your left arm until you can do it without any manipulation from your shoulder, arm, elbow or wrist. Then incorporate the same idea with two hands on the club. Of course the feeling changes immediately you have two hands on the club, but the principle shouldn’t.