I can tell you from experience that many shooters—probably a majority of them—have never been formally trained on how to properly use a rifle sling as an accuracy aid.
The bad news? He’s using a $110.00 sling.
The good news? You can use the exact same technique with a $15 U.S. GI sling (also variously sold as a “Garand sling” or “web sling”) and it is likely to be just as effective as long as your technique is solid.