" you can open the phone and use the full-size camera with the outer screen, which results in much more flattering imagery than the wide-angle lenses selfie cameras use"

A camera held at arms length will have pretty much the same view of your face regardless of whether it has a wide-angle lens. A wide-angle lens just means the image will include more stuff around the edges, but only if the image/video isn't being automatically cropped to frame your face. The view the camera has of your face isn't more or less flattering unless the distance from camera to face also changes.