Again, the pre-VCII theology manuals and the Catholic Encyclopedia teach the same thing about the Magisterium as LG25, and they classify it as de fide. That alone should prove to Sippo that the doctrine was infallibly taught before VCII, provided he remains consistent with what he accepts as authoritative. But I will indulge our physician friend. Only the college of bishops in union with pope, as occurs in a council, or the pope himself, can define dogmas for the Church, so here their infallibility is clearly taught. Sippo may object that VCI here only asserted the Church's infallibility with regard to the teaching of divinely revealed truths.
Infallibility also extends to the teaching of truths linked to revelation.
Many popes taught the same thing as LG25 on the obsequium religiosum owed to non-infallible teachings. Pius XII, in the passage from Humanae Generis that Sippo quoted in his last rebuttal, asserted it, though he used different wording. Even if LG25 did not merely reiterate past teaching, it does not follow that it is an infallible decree. I defy Sippo to find a Magisterial document advocating this Protestant-like caricature of our Faith.