Excuse me, but the quote itself, word-for-word, attributed to pope Paul VI can indeed be found at http://www.vatican.va/holy_father/paul_vi/motu_proprio/documents/hf_p-vi_motu-proprio_19680630_credo_en.html at #23.
There is a bit of further trouble with the continuance in which you offered;
So who is misquoting who?
What the writer of the author of the article (Greg Durel) included was not a "fake" quote. One could argue that it be a limited quote, or even a quite taken out of context.
That the Vatican documents say one thing, then another, is not the OP's or Durel's duty to explain in detail how the seeming having-it-both ways, so called "Apostolic Letter" accomplishes that feat.
Equating what was cited by Durel as equaling in meaning as “all non-Catholics are damned” were your own words to describe what you perceived as how they both perceived the meanings of the quotes...
Personally myself, I have read enough of those sort of "Letters" to see what it is that is being said, and how one statement or a set of statements are later (particularly since Vatican II) are explained away in one sense, but then recovered with declaration along the lines [of meaning] that not only as in past centuries "all must be subject to the Roman Pontiff", there is claim sought sought to be applied that all Christians are, whether they "know it" or not.
That's good work if you can get it?
Durel quoted an actual sentence from Paul VI.
Then he introduced a fake sentence. The fake sentence is directly contradicted by what Paul VI said in the rest of paragraph 23.
Thank you for that research. Well done!