So one's prior for the truth of Scientology can't be anywhere near as low as one would think if one simply assigned an exponentially low probability based on the complexity of the religion.

If nobody had ever proposed Scientology, though, learning Xenu existed wouldn't increase our probabilities for most other claims that happen to be Scientological. So it seems to me that our prior can be that low (to the extent that Scientological claims are naturally independent of each other), but our posterior conditioning on Scientology having been proposed can't.