CBS still calls them:

documents that were provided by unimpeachable sources.

What in the world would it take for a source to be that solid? They aren’t even trying to hedge. How could they possibly feel so confident?

But then again:

unimpeachable

a. Not impeachable; not to be called in question; exempt from liability to accusation; free from stain, guilt, or fault; irreproachable; blameless;

Maybe it’s the “not to be called into question” part they are thinking of, along with the exemption from accusation.