To expand on what I wrote before, try this out:
TBLS sure contains a LOT of stuff that Horn would have no way of knowing, unless he was tagging along and spying all throughout. But that's not likely. He'd have to be several places at once for one thing. Also, the people who might have told him a bunch of stuff are separated from him at the end of the narrative, so it's doubtful he learned much from Auk, Chenille, Silk, etc.
Horn THINKS that he was embellishing and embroidering these parts of his book. Was he?
Wolfe has all kinds of things in the narrative from many sources, but basically it follows the Gospels and Acts, with Silk as a kind of Christ(like) figure. Go with Wolfe to Christian/Catholic theology for a second. Jesus dies and is resurrected, "marries" the wayward Church represented by Mary Magdelene on resurrection morning, ascends to heaven, sits at the right hand of the Father, and sends the Spirit. The Spirit inspires the apostles to recall precisely what is necessary to write the gospels.
Now, everything except the sending of the Spirit to write the book has been noticed quite clearly in TBLS. I think we're supposed to add this step in also. Consider:
Silk has joined Father Pas in Mainframe Heaven. I.e., his personality has been copied and downloaded into Mainframe.
Mainframe also has all the memories/information from all the other gods. Thus, Mainframe knows all about Auk and Tartarus, Chenille's possessions, Mint's possessions, and anything the gods saw as well. So, Silk has access to all that information.
Silk is now downloaded into Horn, or at least Horn is given precisely those memories that are needed for him to write his gospel. When Horn writes, he is not aware of being guided, any more than the apostles of Jesus were aware of being guided as they wrote. Horn thinks he's writing what he remembers, with some fiction thrown in. In fact, however, he's being guided by whatever the Silk information dump has put into him. That's why he knows so much stuff that by rights he should not know.
Of course, at the end of OBW, Horn seems to be in some else's physical body also, possibly Silk's, and so that situation goes beyond what I've suggested happened at the end of TBLS.
Nutria
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.