Warning: speculation incoming

The camera permission probably allows the app to either access the camera directly, or use a special invocation of the camera app that only gives access to the pictures taken during the brief period it's open. This doesn't convey access to the rest of the camera library. That's why it's a separate permission.

No, the camera and library permissions always have been separate, it's just that Apple's official camera app integrates them. Think about it, one's hardware and the other one's basically software.