This is a big, big point against VII. I have been trotting it out more often, because I enjoy imagining the gobsmacked look of the SSPX-er on the other side of the computer screen, heh heh. No AngelQueen type can give an answer to this question, about how someone like Mother Theresa can be "canonized."
It is not exactly a dogma that the canonization of saints is infallible, but let's put it this way -- it is as close as you can possibly get to a dogma without being a dogma, to the point where it would be worse than rash to deny it.
I used to joke about how Vatican II will one day be convicted on a technicality, like Al Capone, who was widely believed to be gettting away with murder but was finally only convicted for tax evasion... So what? It still put him in jail. Even if it turns out that the teaching of religious liberty in Dignitatis Humanae ( the main point against VII that SSPX and sedes agree on ) is really only a change of discipline and not doctrine, which would be near-impossible to prove, you still can't get around this gallery of false saints they have set up. It's a perfect technicality to take down VII.
There are others too, like the encyclical from JPII's papacy about the "anaphora of Mari and Addai." I know -- makes your eyes glaze over, right? But it's important, because Rome claimed that this rite which has no formula for the consecration of the Host is valid! No form! As we all know, there must be matter and form present for a sacrament to be valid. This is another technicality that will be useful in taking down the anti-Popes, proving they aren't protected by the Holy Ghost. Of course I know that the followers of Abp. Lefebvre will say that papal encyclicals aren't infallible on matters of faith and morals, but this is to deny yet another dogmatic fact.
Do you see how, in order to have their precious unity, they are gradually whittling away the immaculate bride of Christ to nothing? At this point the "Church" triumphantly claimed by SSPX to still reside in Rome, the one they trumpet their unity with, basically rests on two ex cathedra statements in two hundred years, and everything else can go to pot... Yet the sedes are considered the ones who are extremists? What kind of "Church" is this, SSPX?