Zephiel7, I don't recall the issue appearing in PoD, but BotS.
@Glentract, their atmospheres were no longer as close as they were thousands of years ago, and Bane was confirmed to have flown via a beast across the atmospheres, and the only explanation is that he pulled the moon out of orbit, otherwise it wouldn't have been possible. Anyways, here's the exact quote, draw whatever you want from it:
'Now that he understood the depths of Sith powers, he thought he might even have control over worlds and moons, able to play with orbits and gravity like a child might play with colored balls.
Long ago, Dxun had grazed the planet Onderon, close enough that it was possible for creatures to pass across the conjoined atmosphere. Perhaps Bane could nudge the beast moon close enough so that he could travel to the nearby planet that filled the sky. In bloodshed and chaos, Darth Bane would go to Onderon ... and there he would find his apprentice. '
The fact that he believed himself powerful enough to do so alone kinda speaks volumes, Bane was never arrogant of his powers, to believe that he would have that kind of power is pretty incredible.
@AC, most of those alternatives you gave aren't actually valid, he can't have just found a ship or teleported because he was actually confirmed to have flown on top of a beast across the atmospheres, and the only way that would have been possible is if the atmospheres of Dxun and Onderon were brought close enough together.