I doubt it.
It looks as if it has an atmosphere and you can make out clouds. But it seems to be in a really familiar place.
The Nebula makes me think this is indeed Threshold.
Well if you look at the concept art trailer, you see a planet with atmosphere, so it’s not impossible that the one in the background is the one from the game. In plus, take a look at the shield world in Halo Wars, it had an outer atmosphere as well as an inner one.
In plus, we never got a good look at it at the end of Halo 3, so it could be anything.
Edit : Actually, I just watched the Halo 3 legendary ending, and the nebula shown there looks a lot like the one in the background.