Honestly, that's not needed if entering elbow first into a loose shoulder strap. If you're putting your harness on like a coat, reaching backwards and fishing for the arm strap... yeah, that's harder than necessary and would indeed require mobility/flexibility or a way to really loosen up the strap (i.e., the device you linked). The video I linked demos what I'm talking about, albeit underwater. On land, you can insert the right elbow second and hold the long-hose (attached to the right d-ring) to give yourself a "hole" for the elbow.