It seems to me that a critical point is to state the new Rites were legitimately promulgated.
(that we didn't know about before)
That is hidden by the smokescreen of admitting they are valid -
The Church cannot legitimately promulgate harmful rites.
So if the SSPX accepted this then it would mean it implicitly accepts the Novus Ordo.
Just as the docuмent admits that Vatican II is implicit in Church Tradition.
And gives to Vatican II the level of assent required in Lumen Gentium.
This religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will. His mind and will in the matter may be known either from the character of the docuмents, from his frequent repetition of the same doctrine, or from his manner of speaking.
So Vatican II is admitted to be authentic magisterium, the new rites are admitted to be "legitimately promulgated"
So to accept the preamble is to accept Vatican II and the New Mass.