Urth/Briah is an oscillating universe preceding our particular oscillation. That is why Severian's book can (somehow) survive and be translated by Wolfe.
Wolfe is assuming that each oscillation pretty much duplicates the one preceding it, with perhaps some advances, as each Briah race becomes the hierodules for the next cosmos. Thus, for all intents and purposes, Urth is Earth, and Severian is in "our" future.
From things Wolfe has said in various interviews, including mine, I think we should take this a given.
Nutria (Jim Jordan)