One troublesome aspect of this statement, in my opinion, is the following:
Indeed, it is because of the serious failings and omissions of the Novus Ordo Missae and of the reforms introduced under Paul VI that the Priestly Society of St. Pius X seriously questions, if not the validity in principle, then at least the “legitimacy of the Holy Mass or the Sacraments celebrated in the forma ordinaria” (n. 19), since it is so difficult, as Cardinal Ottaviani had already noted in 1969, to consider the Mass of St. Pius V and that of Paul VI to be in the same “apostolic and unbroken tradition” (no. 3).
It seems to me that the statement is misconstruing the meaning of the term "legitimitatem" in the instruction. It refers to the legality of the novus ordo, not the difficulty in considering it an authentic development of Catholic worship. There is a clear distinction between the two concepts, one being purely legal in nature, and the other being analogous and moral in nature. There can be no question that the pope has the legal right and authority to promulgate liturgical law. It is another question whether or not he has a moral right to do so in given cases and circumstances, or whether any given liturgical law promulgated is a prudent act.