It was an ex cathedra pronouncement? I believe that its infallibility comes as a result of it stemming from the ordinary magisterium of he Church, not from a papal pronouncement.
I think that’s in reference to disciplines. Disciplines aren’t infallible doctrines, but we all have to adhere to them in order to all be on the same page when worshiping God. For example, imagine if at Mass everyone would sit, stand, or kneel whenever they felt like it instead of at certain prescribed times during the Mass. There would be a lot of confusion going on which would distract from the celebration of the Mass.
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