"We teach and define that it is a dogma Divinely revealed that the Roman pontiff when he speaks ex cathedra, that is when in discharge of the office of pastor and doctor of all Christians, by virtue of his supreme Apostolic authority, he defines a doctrine regarding faith or morals to be held by the universal Church, by the Divine assistance promised to him in Blessed Peter, is possessed of that infallibility with which the Divine Redeemer willed that his Church should be endowed in defining doctrine regarding faith or morals, and that therefore such definitions of the Roman pontiff are of themselves and not from the consent of the Church irreformable." [Catholic Encyclopedia - http://www.newadvent.org/cathen/05677a.htm]
Oooops.
I also as a Catholic was confused by Campion’s post. I think Campion misspoke there/didn’t write what he meant. I think he is referring to the non-essentials of the document (such as wording for example). Although the main issue regarding faith and morals is divinely revealed and therefore free from error it does not mean that the human who wrote it chose the perfect wording to explain it.
Hopefully he can come in and shed some more light on this, but I think this is what he’s driving at.