Yes, technically, it could just be a moon, but during formation, it would have to have sufficient mass to be able to hold on to its own atmosphere. Too little, and the lighter gases would get sucked away to the bigger partner, leaving it a barren rock.
That thing in the sky really looks like it has a substantial atmosphere - which means enough mass to hold onto it, so it probably is comparable in mass to Rivellon.
Earth is 81 times more massive than the Moon, and even then the gravitational center of the Earth-Moon system is about 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.
There's also a valid argument that the Moon orbits the Sun (other satellites orbits sometimes have them move away from the Sun, but the Moon always moves towards the Sun), but that's debated and a bit beyond the scope.
