The Death Star is so massive that it is capable of warping space and time, making it capable of time travel. The partially constructed Death Star in ROTJ is actually the same one from ANH. There is a universe in which it was completed after ROTJ and sent back in time, and that’s the universe we see in ANH, and it’s also why everyone in the galaxy was blindsided by a moon-sized object being built out of nowhere. In this headcanon, the Death Star would have started construction only a few years before ANH, leading to the station we see in ROTJ.