The document “Humane Vitae” is not infallible. Its main Idea is! here is the thought of Cardinal Mueller on the main teaching of the document.
"The Archbishop of Krakow sharply distinguishes two things: he distinguishes a dogmatic definition ex cathedra which is not given in Humanae vitae, and the presence of a teaching that is affirmed in Humanae vitae with the authority of the ordinary universal magisterium.
And since the Archbishop of Krakow thinks, on the basis of what Humanae vitae itself states, that the moral teaching of the Church is not an arbitrary decision, that the Magisterium is not an exercise of power but is a service to the truth, the Archbishop of Krakow is convinced that the teaching of Humanae vitae expresses a truth about the good, which the Church does not have the right, does not have the presumption to define or to change, but of which she is simply the depository and witness.
In this sense, the letter of 1969 simply invites Pope Paul VI to reconfirm what the encyclical Humanae vitae says, that is, that the teaching belongs to the universal ordinary magisterium, which according to the teaching of the Second Vatican Council in Lumen Gentium, 25, is infallible. Therefore, not the definition ex cathedra of a new dogmatic truth according to the teaching of Vatican I, but rather the acknowledgment of the truth as ordinary universal magisterium of what is written in Humane vitae itself."