This was first shown to be a forgery in the 15th century, by a "Roman" Cardinal/literary detective.
It's not something "Rome" had to be forced to "admit" by an American gal with a fake name skidding into a website almost 600 years too late.
It appears that Rome was forced to admit they were forgeries, only after they could no longer prop them up.
The chief affair was the maintenance of the authority of the False decretals, Gratian, and the forgeries accepted by St. Thomas Aquinas. For a long while no one in the Catholic Church dared to expose the latter. French scholars were the first, about 1660 to tell the truth about them. Gratian's Decretum had gained new authority through the revision and correction ordered by the Popes, in the course of which many forgeries must doubtles have been detected. The pseudo-Isidore was still for a long time protected by the Index. When the famous canonist, Contius, brought forward the evidence of its spuriousness, the Preface in which this contained was suppressed by the censorship.
On the appearance of the famous work of Blondel, which completely dissected the pseudo-Isidore, the last doubts about the true nature of the fraud were exploded. But it too was placed on the Index. About the time of the Declaration of 1682, the Spanish Benedictine, Aguirre, made the last attempt worth mentioning to rehabilitate the pseudo-Isidore. It could now no longer be denied that with this forgery disappeared the whole historical foundation of the papal system for any one acquainted with history. Aguirre was rewarded with a cardinal's hat. But in the course of the eighteenth century it came to be perceived at Rome that it was impossible to maintain any longer the genuiness of this compilation, and thus at last the fraud, was admitted in the answer given by Pius VI., in 1789, to the demands of the German archbishops.