A “Secret archive” that has been open to researchers since 1980 isn’t so secret. It’s essentially a trove of documents so large that the Vatican lost track of what they had, so they allowed outside researchers to rummage through their “attic” to see what they could find.
I just find this interesting; it has nothing to do with the article since they mean “secret” in the modern sense:
The modern usage of ‘secret’ comes to us via the mass. Initially, the word meant “set apart,” as opposed to collect, which, of course means, “gather together.” The Collect of the mass is the prayers said with the priest and congregation in unison; the Secret of the mass is the prayers said by the priest on his own. Thus, secret came to mean something said privately, in quiet tones.
So basically Vatican’s Private Library (even though that is redundant) is really what is meant?