just that souls must exist in some way between bodies.

Not even that, at least in the part of the argument I’ve seen (paraphrased?) above.

He just mentions an ancient doctrine, and then claims that souls must exist somewhere while they’re not embodied, because he can’t imagine where they would come from otherwise. I’m not even sure if the ancient doctrine is meant as argument from authority or is just some sort of Chewbacca defense.

(He doesn’t seem to explicitly claim the “ancient doctrine” to be true or plausible, just that it came to his mind. It feels like I’ve lost something in the translation.)