[SECGolf]

Wouldn't too much extensor action impede a player from swinging the left arm freely from it's socket?

I would not think so. You are only stretching the left arm via the right hand.
Stretch all you want but the joint structure will still be in place. The shoulder joint has no choice but to do its thing. What would impede the left arm is lack of extensor action - with lack of, the left arm would collapse - centainly discouraging a swinging left arm. Also, a non-inert left arm would certainly interfere with the natrual action of the left shoulder joint.

With extensor action the left arm is acted on.

Stretch the leash as much as possible (even if made out of a steel beam), if attached to a hinge, it will still swing, but with more structure, it will resist collapse, especially when something at its end hits another object (say for example, a golf ball).