I think that the problem is that EY has introduced non-standard terminology here. Worse, he blames it on Jaynes, who makes no such mistake. I just looked it up.

There are two concepts here which must not be confused.

• a priori information, aka prior information, aka background information
• prior probabilities, aka priors (by everyone except EY. Jaynes dislikes this but acquiesces).

Prior information does indeed constitute a hypothesis in which you have complete confidence. I agree this is something of a weakness - a weakness which is recognized implicitly in such folklore as "Cromwell's rule" Prior information cannot be updated.

Prior probabilities (frequently known simply as priors) can be updated. In a sense, being updated is their whole purpose in life.