That took two days to parse, but now I understand how it works. You're right. I apologize to everyone for having defended an incorrect position.

My misconception seems to be popular, though. Maybe someone should write a toplevel post on the right way to think about the universal prior. Though seeing that some other people are even more hopelessly confused than me, and seem to struggle with the idea of "prior" per se, I'm not sure that introducing even more advanced topics would help.