That's not a decline no matter who says it is.
If you face forward, it's an incline. If you face backwards, it's a decline.
If you were standing up and your back was angled in that position, you would be described as bending over, not facing upwards. I don't even see how this is debatable.
Regardless of semantics, if you google image "decline dumbbell curls", the first three images are this exercise. The top ten are half of this exercise and half of incline dumbbell curls, so that is one of its common names. The fact that there is even debate over what it is called reinforces my original point about it not being something you'd learn in the first month of going to a gym.