And the reason you think that the external rotators are slow in coming back is because you are being told that they function in isolation as external rotators of the humerus. They do not. The only time they actually do this in in the PT office. Think for a second: have you ever performed this motion outside of the therapy appointment? Ever? Even once? Their actual anatomical function is as humeral stabilizers -- they maintain tension on the humeral head, keeping it seated in the glenoid. They also
happen to externally rotate the humerus if you construct an artificial situation in which this can be done with a tiny weight -- that cannot make anything stronger.
Presses and chins require that the humeral head remain stable in the glenoid, and all the muscles that perform this function are involved in these movements. ALL OF THEM. So, if you press and chin/lat pull, you are working all these muscles, including the injured ones, and doing so in a more normal anatomical context than in isolation with a 3-pound dumbbell or an elastic band. In the systemic context, the injured muscles work while (whilst?) being protected by the healthy muscles adjacent to the injury. As they heal, they assume a greater share of their normal anatomically-assigned role. And this is the basis of correct rehab. Typical isolation-based PT is not.