No, it is not security through obscurity. It’s a message signature algorithm, which are used in cryptography all the time.

Yes it is. The scheme is that when you take a picture, the camera signs said picture. The key is stored somewhere in the camera. Hence the secrecy of the key hinges on the the attacker not knowing how the camera accesses the key. Once the attacker knows that, they can get the key from the camera. Therefore, security hinges on the secrecy of the camera design/protocol used by the camera to access the key, in addition to the secrecy of the key. Therefore, it is security by obscurity.