The same exercise was suggested to me when I was seeing the physio a couple of years ago; my other half did the exercise to me. It did help to some extent but when the exercise was repeated with the arm a little further away from the body it caused the most massive flare up on several occasions when tried (under supervision from physio). The approach of my physio was to repeat a series of stretches like those you describe, starting with the arms very close to the body then moving the arm further away because this increases the load on the muscles and if successful should have restored flexibility. After nine months of trying and failing to move beyond the basic position she gave up. My lumps and bumps remain but I’ve passed the point of noticing those pains, I have other more demanding ones I’m afraid and when they do flare up aromatherapy, hot water bottles, sleep and lots of shouting seems to help restore a manageable but unpleasant level of pain.