If the minions of long dead and excommunicated Archbishop Marcel Lefevre's vicious little anti-papal schism want to be regarded as Catholics, they simply need to unconditionally surrender and submit to papal authority.
This is true of all who sin. When the priest in my Church insists that he is above the Church's law regarding the liturgy and can act on his own initiative to change the Mass, or add to it, or delete from it, he is also responsible to "unconditionally surrender and submit to papal authority." However, he remains Catholic even when he sins. So do the SSPX.
The SSPX are "Catholic" only in the same sense that the Catholic baptized Fr. Martin Luther or Jean Cauvin were (i.e not at all as long as unrepentant).
There is absolutely no comparison. Martin Luther openly denied the Catholic faith and Church, and intentionally set up a new institutional Church. The SSPX have not done this. Sure, they are wrong, and their actions have been schismatic (though the competent authorities in the Holy See have repeatedly insisted they are not formally in schism) but they are not setting up a new institutional church as did the Protestants. There is a vast difference here.
Dario Cardinal Castrillon de Hoyos is not and, God willing, never will be pope and he has no personal authority to countermand the papal JUDGMENT, not mere opinion, of John Paul II (who had been his patron and whom he has betrayed).
I am uncomfortable with this view which is actually very like that of the SSPX. The above Cardinal, and the commission he headed, were the competent authorities regarding the traditionalist adherents for the Holy See, and this included the SSPX. Unless Pope Benedict XVI were to openly deny what they said in reference to that which they are legitimately qualified to address then one is bound by faith to accept their word as his own. If your view were correct then one could freely ignore the Congregation of the Doctrine of Faith regarding questions of faith or morals, or the views of the Congregation for Divine Worship regarding the proper forms for liturgical acts. And I guarantee you a Catholic is free to do no such thing.
For what it may be worth, I am no fonder of Novus Ordo Masses than you may be. Nonetheless, they are legitimate Masses whatever Marcel's minions may imagine. I attend only Tridentine Masses said by the Institute of Christ the King and its local pastor with the blessings of Bishop Thomas Doran of Rockford who has long since given his permission as diocesan ordinary to ALL of his priests to say Tridentine Masses in appropriate venues.
For what it is also worth, I have never once in my entire life seen or participated in any Mass but that which you call the "Novus Ordo." I have no attachment to the old forms, though intellectually I have opinions regarding them and would certainly welcome an opportunity to attend and experience it. My position is entirely free from any personal ideas about the issues, but is entirely concerning the objective claims that members of the SSPX are not Catholic, and that they are anti-semites. The first is wrong on the facts, and the second cannot be substantiated, which causes it be dangerously close to calumnious gossip, IMHO.
If you have never attended a Tridentine Mass you are far younger than I or a once indifferent Catholic or a convert. You may well be a Catholic today but I find most curious the notion that B-XVI's alleged "silence" in the face of Castrillon de Hoyos's brazen attempt WITHOUT AUTHORITY to ignore the PAPAL ecclesiastical judgment of John Paul II (the pope that the Lefebvrites most love to hate because he excommunicated and declared schismatic their anti-papal and anti-Catholic heroes in schism) especially since B-XVI himself, as Joseph Cardinal Ratzinger, joined with John Paul II in excommunicating the virulently defiant SSPX bishops and their adherents in 1982 as they had stolen the power to consecrate bishops in DIRECT DISOBEDIENCE to JP II.
That B-XVI has not seen fit to publicly comment on Rembert Weakland's homosexual abuse of seminarians while Archbishop of Milwaukee does not suggest papal approval of such behavior.
No one whomsoever in SSPX has any authority whatsoever to grant faculties to priests (ordaining them illicitly is one thing and granting faculties quite another/ask any suspended priest) and yet the SSPX miscreants hear non-emergency confessions, and witness to non-emergency marriages in dioceses with diocesan ordinaries without so much as a by your leave to actual authority. This makes them quite analogous to Luther and Cauvin and they are, in fact, setting up their own Church. Read their rationalizations as to how the Roman Catholic Church would die out for lack of actual priests if Marcel the malignant did not directly defy papal orders to consecrate bishops against JP II's direct orders. Read their newsletters and publications. Read particularly de Mallerais whose vile mouth makes Williamson look about as scary as Soupy Sales or Pee Wee Herman.