Yes, you are correct regrading nearsightedness and detachment (I speak from personal experience having torn, not detached, my retina). The tear had nothing to do with firearms but, I was advised to watch what I was shooting in the future because tears can lead to detachments.
The MD's point was simply that near-sighted people and as people age (both have thinner retina's), and people who have experienced head trauma (falls, hockey, football, boxing, etc.) have an increased chance of tears/detachments. It has nothing to do with your ability to handle recoil, it is a question of whether your retina and the viscous fluid in the eye can. If your fluid is too "sticky" (retina and the fluid) and/or you have had a sharp blow to the head in the past, transmission of additional "shock" could cause a tear and a tear a detachment.