If you look closer, you can see that the plastic area inside of the s-pen holster is plastic and comes out a bit and is met by the "metal" trim. What you are calling the area where the "metallic paint" ends, actually looks identical to the spots on the top and bottom of the phone trim that are broken up by plastic, probably to prevent excessive warping. If you take off the back of the phone, it's obvious that it's not structural in any way, regardless. Whether it's a thin metal strip or metallic paint is irrelevant as far as quality and durability goes. For the record, it looks and feels like a thin metal strip.
I'm not going to take a hammer to my new phone to prove who's hypothesis is correct, but I take issue where you claim your photo is irrefutable proof that there is no external metal.