His secretary wrote that these are Hitler's communications. Absent evidence to the contrary, it must be assumed they are legitimate. Nothing is needed to "back them up." How does one "back up" the communications records of an historical figure? Unless the exact correspondence or communication can be found by the receiver, that is a near impossible task in any situation. Given that criteria, almost nothing purported to be said by Hitler can be unqualified documentation as everything is a "secondhand account" unless it was actually written in Hitler's hand. But, Like Winston Churchill, Hitler dictated almost everything to a secretary. The secretary's records serve as the primary sources.
Borman's records are evidence in themselves.
Welcome to the world of historical documentary evidence.
"Certainly, the record of someone's private secretary taking dictation for correspondence or other communication qualifies."
But that isn't what this is.
That is precisely what they are.
I assume by your screen name that you are a guitar player. What do you play?