To be fair to the saintly pontiff, Pius X, if he had read the works of Newman himself, he would have no need to the defer to the letter written by the Bishop of Limerick. Most popes are too busy writing encyclicals, writing their own tomes, than to busy themselves with reviewing the literature of others. My guess is that Pope St. Pius X endorsed Newman on account of his "English convert" status, as well as his voluminous authorship. Most popes rely upon the theologians around them to critique the theological writings which are of questionable orthodoxy, and this is understandable given the various responsibilities to which the pope is subject.
We all know that the modernists were thick in the church even before Cardinal Sarto entered the papal office; in fact, the freemasons in Rome cheered when Giovanni Mastai-Ferretti was elected in 1846, as Pius IX. Thankfully he proved to be their enemy, but it cannot be doubted that the spirit of modernism was prevalent even under the orthodox popes of the nineteenth century.
Perhaps without realizing it, your post is tantamount to accusing St. Pius X of irresponsibility, dereliction of duty, negligence, and according to the opinions of some here, being an unwitting dupe of modernism, and working against himself by promoting modernism.
No, I think there was sufficient controversy surrounding Newman for the pope to have done his due diligence, investigated the arguments of the Irish bishop, and concurred.
If someone wants to argue otherwise, the onus is upon them to provide the proof.
I would also be remiss in failing to note the irony of Newman’s CI critics being guilty of the very thing they have the temerity to accuse St. Pius X of (ie., relying on the opinions of others without having ever read Newman themselves).