When people ask for independent confirmation of Biblcal accounts, they aren't asking for Christian material that didn't happen to get included in the Biblical canon. Peter was never part of the Bible, but it was definitely written by either the same communities that produced the New Testament, or by closely related sister communities. It is true that as you yourself say, there is a large volume of Christian material that didn't make it into the Bible, but they were all produced by Christian communities.
Historians like Josephus and Pliny are examples of source from which independent confirmation can sometimes be drawn.
--Percy