A teaching may be de fide, that is dogma, without every iteration of that teaching by the pope being ex cathedra. In Ordinatio sacerdotatalis we have a statement of a truth that is de fide, dogma. But according to the CDF it was not an extraordinary exercise of the papal magisterium, not ex cathedra, but the teaching is irreformable and infallibly taught in virtue of the ordinary and universal magisterium.
(I know there is a fuller treatment of it by then Cardinal Ratzinger, but cannot find it at the moment)
The same can be claimed of the teaching, in respect to the impermissibility of contraception, in Humanæ Vitæ. (Note, even in bulls, encyclicals, etc that have an ex cathedra definition of a truth of the faith, not everything therein is necessarily part of the ex cathedra definition, or even part of the irreformable teaching of the Church. Hence certain arguments, lines of reasoning, other teachings touched upon, do not become irreformable teaching merely by being part of the same writing). E.g. the approval of NFP, while one isn't simply free to disregard it, is not on the same level (this is clear because it had never been accepted definitively by the ordinary and universal magisterium. Papal declarations, short of a definitive answer, not only add weight to believing it is licit, but are authentic pronouncement of the ordinary magisterium, that are owed "obedience of will and intellect", but do not, by themselves, raise it to something de fide ecclesiatica (that is an irreformable truth, but not one resting on revelation. The fact is that most moral teachings, in themselves, cannot, by their nature, be dogma in the sense that it would be heresy to deny them, as they are truths of natural reason. Of course, some truths of reason are also revealed, and perhaps that could be the case with contraception, c.f. Onan)
It does seem to me that Ordinatio sacerdotalis does contain an ex cathedra declaration. However, what is claimed by the CDF, with approval of the pope, is that it isn't, but rather the teaching was already irreformable. I suppose the thought is that it didn't become irreformably defined at that instant, it does not owe its "infallible" nature to that specific declaration.
One should also note that there are teachings that are not dogma/de fide, i.e. the denial of them is not heresy, yet must be held definitively. Such teachings are not objects of the faith per se, but are irreformably taught by the Church as either supposed by the faith, or following therefrom (it is a disputed question whether a truth that is demonstrably certain from two premises, one that is a matter of faith, the other of natural reason, is itself a matter of faith. The argument has more than 2 answers, with some holding that it is only if the major premise is the de fide teaching)