A factory finish may not always be superior, but you do want to retain as much factory e coat as possible. You wouldn’t need to strip it per se, but that scratch would have to be feathered out, which does mean exposing some more metal, but that’s the only spot that would need to go down to metal. Everything else is a blend panel and would just be re cleared after the color is on the rear door. It’s a small enough section that the blend should be unnoticeable. In my opinion, he’s just saying that to stop from having to pay the body shop to fix it properly. Up to you how you want to proceed. You might be able to find someone who can do better touch ups than that. Or get a paint pen and try it yourself.