It means exactly what it says. It says that the douay is the only bible declared to be free of doctrinal errors by a council. There are errors in other translations.
The Douay predates the KJ.
For the historically dysfunctional the catholic church was here long before there was a bible. The catholic church codified the bible in 397 at the council of Carthage. It was there it was decided on which 72 books would make up the bible(45 OT,27 NT)
At the reformation the protstants took out 7 books(Tobias, Judith, Wisdom, Ecclesiasticus, Baruch, and I, II Machabees) and parts of books (viz: Esther 10:4-16:24, and Daniel 3:24-90; 13:1-14:42) which are not found in the Jewish editions of the Old Testament
These 7 books,however, were only removed by the Jews in 100 A.D. at the council of Jamnia.They removed them because too many converts were being made to christianity from them. So the protestant reformers based their selection of books on an anti-christian council.(Jews in other parts of the world eg Africa kept them).
It was the catholic church that gave people the first bible in English.The Bible In English (before printing)
Various parts of the Bible were translated into Saxon, the language of England at that time, by:
c. 670 Caedmon, a monk
c. 709 Aldhelm, Bishop of Sherborne
c. 735 Venerable Bede, a monk of Jarrow
c. 849-901 King Alfred
c. 955-1020 Aelfric, Archbishop of Canterbury
c. 1020-15thC. English, as we know it today, was developed during this time, and translations appeared in the language of the time. (prior to the reformation there were no protestants
1454 A.D. THE FIRST PRINTED BIBLE: A Catholic named Gutenberg caused great excitement when in the fall of that year he exhibited sample pages at the Frankfurt trade fair. Gutenberg quickly sold out all of the 180 copies of his Latin Vulgate Bible even before the printing was finished.
THIRTEENTH CENTURY, THE FIRST DIVISION OF CHAPTERS: It was the British Catholic Arch-Bishop of Canterbury, St. Stephen Langton (died 1228), was first to tabulate scripture into Chapters, and we follow his arrangement to this day: some 1,163 chapters in the Old Testament, and only 260 in the New Testament."
1551 A.D. THE FIRST DIVISION OF VERSES: The first division of the Bible into its present verses is found for the first time in an edition of the Greek New Testament published in Paris by the Catholic Robert Stephens
1555 A.D. THE FIRST PRINTED COMPLETE BIBLE WITH CHAPTERS AND VERSES: The first division of the Bible into its present chapters and verses is found for the first time in an edition of the Vulgate published in Paris by the Catholic Robert Stephens
Douay..translated from the Vulgate by St. Jerome in 1609
KJ was translated 1611
And I suppose all the hundreds of theologians who have poured over the Greek and Hebrew texts from the Vatican to develop some of the newer versions like the NASB are in error as well. The Vatican tells you what to believe and you accept. It does make life simple.