The issue is the Vatican Council did not clearly outlie what "Ordinary Universal Magisterium" means. I am unsure about the Pope question at the moment.
Everything was explained in great detail, but it was explained during the official relatio delivered during the Council and in the questions and answer that followed; not in Dei Filius itself. There are three things to consider:
1) The teacher
2) The doctrine taught.
3) The mode of teaching.
1) The Ordinary and Universal Magisterium is the
teachers, and as Bishop Martin explained during the Council, it consists of all the bishops dispersed throughout the world.
Not the bishops gathered together in a council (and there is an important reason for that), but the bishops dispersed in their diocese.
2) According to the teaching of De Filius, the
doctrine taught is a revealed truth. That doesn't mean the infallibility of the OUM can't apply to a non-revealed truth, but it does mean that the teaching of Vatican I pertains only to revealed truths.
3) The part that hardly anyone understands is how the doctrine must be
taught to be infallibly by virtue of the OUM. It must be proposed in a definitive and absolutely binding manner, in such a way that all the bishops believe it to be a dogma, even though it has not been solemnly defined by a single act of the Church. The doctrine in question must be one that is owed the unqualified assent of
divine and Catholic faith. That is the level of assent that is
only owed to dogmas. This greatly limits the subject matter that is infallible by the force of the ordinary and universal magisterium.
An example of truths that are infallible by the OUM are 1) that Christ was transfigured on Mt. Tabor, 2) that he sweat blood in the Garden of Gethsemane, 3) that He said "Father into thy hands I commend my sprit" before dying on the cross. None of these revealed truths have been solemnly defined, yet they are truths that the Magisterium has always taught as literal truths, as written, and which all Catholics must believe with the assent of divine and Catholic faith.
The reason the Council taught this doctrine concerning the OUM was to counter an error known as dogmatic minimalism, which emerged during the previous century. According to this error, only what the Church had solemnly defined required to assent of faith, and nothing else. If that were the case, only the teachings of Scripture that had been solemnly defined would be part of the Catholic
rule of faith - that is, they alone would constitute the body of doctrines that all Catholics must believe by faith. That is clearly not the case.
What nearly everyone mistakenly believes, including Fr. Cekada and, from what I have been able to gather, ALL sedevacantist priests, is if any doctrine is generally
accepted by all the bishops, or if a doctrine is addressed by the pope to the universal Church, it meets the conditions for infallibility by virtue of the Ordinary and Universal Magisterium. This is entirely false and it is one of the principle errors that leads people to embrace the sedevacantist error.
Only the undefined doctrines that the entire Magisterium agrees must be believed with
divine and Catholic faith meet the conditions for the infallibility of the OUM.