I've some extra time, I'm trying to print that whenever we speak about internal external shoulder rotation for most neutral might already be internally rotated and if one gets into issue it may be a good idea to experience (and discover) the full range of motion of the arm in those opposed rotations ( external arm probation for the forearm). It felt weird to me but got me realize I never did a GOOD push-up. Then clearly the scapula can move a lot and internal rotation is necessary ( but most do not start from neutral and there is a lot of strength to be found by forcing external rotation).
I believe Phelps does it right thought at racing pace one does not have the time for perfect motions so he short cut to increase his spl. Phelps shoulders , flexibility aside which is linked, are not rounded at all and it back is relatively flat too. Many swimmer have rounded shoulders and back, he might do something different so his back muscles are not overwhelmed by his pectoral/front muscles.