Actually, it didn't approve the so-called "Secret" of La Salette. In fact, the "Secret" was banned by the Holy Office:
Wednesday, 9th May 1923In a General Session of the Supreme Holy Congregation of the Holy Office, eminent and reverend Lord Cardinals appointed for protecting the faith and morals, proscribed and condemned the little work The Apparition of the Most Holy Virgin on the holy mountain of La Salette, Saturday 19 September 1845. - Simple Reprinting of the entire public text by Mélanie, etc. Society Saint-Augustine, Paris-Rome-Bruges, 1922; ordering those to whom it looks to take care to withdraw examples of the condemned book from the hands of the faithful.
And the same holiday and day of the Most Holy Lord. N. D. by the foresight of divine providence Pope Pius XI, in the customary audience of R. P. D. Assessor of the Holy Office has commissioned the report himself and approved the resolution.
Given at Rome from the Holy Office on 10th May, 1923.
Aloisius Castellanus, S. R. and U. Notary.
Similar to the phony third secret of Fatima...can't approve of the apparition and then deny the message. That would be like going to the Novus Ordo but claiming to be traditional or orthodox.