The Pope is talking nonsense. But you actually left out the good part:
"Moreover, I should like to remind theologians and other experts in the ecclesiastical sciences that they should feel called upon to answer in the present circumstances. Indeed, the extent and depth of the teaching of the Second Vatican Council call for a renewed commitment to deeper study in order TO REVEAL CLEARLY THE COUNCIL'S CONTINUITY WITH TRADITION, especially in points of doctrine which, perhaps BECAUSE THEY ARE NEW, have not yet been well understood by some sections of the church."
So the Pope is saying at one and the same time that the Vatican Council is in continuity with Tradition--but that it teaches doctrines that are new. How's that for clarity? Since when is Tradition something brand new? And since when are these doctrines so difficult to understand that they require study by experts to show their connection to Tradition? Seems to me the Pope is looking for somebody to force the square peg of modernism into the round hole of Catholic Tradition! He wants experts to do this. Yeah, sure. Plain language isn't going to be able to, especially when it comes to doing away with the principle of non-contradiction.
And while we're at it, since when does the "living" Magisterium cancel-out previous teachings of the Church? The Pope and Council can have no divine protection for what they say except when their doctrines are in complete conformity to past teachings. So why the emphasis on the "living"--unless the Pope is trying to get us to think that novelties bind us in the same way as traditional teachings? But they don't--which was the Archbishop's whole point! And when novelties are in flagrant contradiction to past doctrines, they are simply falsehoods that must be rejected! So once again the Pope has shown it is HE, not Archbishop Lefebvre who doesn't speak accurately when it comes to Tradition.
He's talking about "points of doctrine" which are new - that is, the statement on religious liberty, which was Msgr. Lefebvre's main problem with the Council.
You do realize that the Church's authority and infallibility extends to truths necessary to safeguard and expound the deposit of faith, including the natural law, right? That is, the Church can infallibly teach things which are not contained at all in the deposit of faith. That is why the Pope can say there are new points of doctrine - because the Church had reached a deeper understanding of religious liberty (compare Ci Riesce with Dignitatis), which was an authentic development of the previous teaching:
There is no corruption if it retains one and the same type, the same principles, the same organization; if its beginnings anticipate its subsequent phases, and its later phenomena protect and subserve its earlier; if it has a power of assimilation and revival, and a vigorous action from first to last. (Ven. Cardinal Newman, Essay on the Development of Christian Doctrine)
Yes. What was the church thinking, giving out doctorates in Sacred Theology? Church doctrines are all so easy to understand that we don't need any experts - just our own private judgment inspired by the Holy Ghost.
since when does the "living" Magisterium cancel-out previous teachings of the Church?
It doesn't.