Actually, come to think of it, they were still in their ships, weren’t they? Maybe a backdrop of Sullust then…
The entirety of the Rebel rendezvous takes place on the Rebel Starcruiser. There’s no planet visible, and although Vader says they’re massing near Sullust, “near” in the grand scale of space can 100% mean that no planet, or even the star Sullust orbits (or maybe Sullust is the star), should appear. On the galactic scale, “near Earth” could mean somewhere beyond Pluto. The lack of any notable stars or planets in all of the exterior shots of the Fleet enhances the sense of isolation and attempted hiding from the Empire.