I missed this on the first reading, I find it interesting:
Le Rocher: Are these studies – and the publications[2] based on them- still up-to-date?
Bishop Fellay: By looking again at all the studies we have publish, we can show that these canonizations are not serious, even if there is still the famous problem of the infallibility that would be implied by a canonization. Know that this is a point that can still use work. On matters of Faith, the infallibility of the dogmas, there is no room for discussion. But as for the infallibility of canonizations, as they are not the primary but rather the secondary object of the infallibility, there is still room for discussion.
From what I have read that the SSPX has put out so far regarding the doubtful post Vatican II canonizations, this seems new. I have never heard it mentioned that the infallibility of canonizations as such was under dispute. The argument has been that the changes to the process have rendered it doubtful. Has anyone read anything from the SSPX suggesting this idea? My understanding of canonizations (limited as it may be) is that the majority/probable opinion among theologians is that they are infallible in virtue of the pope making a solemn declaration and binding the Church. Because of the nature of the liturgy and universal worship of the Church, she could not make a mistake in canonizing someone and having the Church over Mass for someone who could be in hell. What the minority opinion is and why they don't consider canonization infallible as such, I am not too sure about, I am certainly no theologian. But as I understand it the Church has never ruled on this definitively either way.