The NT as a whole is from so many different sources that it would take all day to discuss. I'll just place my bets with St. Jerome that the included books are Canonical and inspired.
If you hold that the rest of the NT is subsumed to the history reported from the Gospels and the Acts of the Apostles then then whence doctrine ?
And if docrine is really doctrine then the authorship of those other books would be germane to the kernel, ?
I assume the author if this letter is genuine unless proven otherwise. Did you go to the website and study it for clues ?