Sounds to me like they’ve drifted from schism to heresy. I wouldn’t be surprised if they continue to separate into more and more diverse groups with ever differing interpretations of what Tradition teaches as they have opted for the very Protestant “I’ll start my own thing when the Church does something I don’t agree with” approach to Church authority. Ironically, SSPX takes a hard interpretation of
Extra Ecclesiam Nulla Salus and they are themselves
extra ecclesiam. I love the Extraordinary Form - nothing does more to hurt the spread of this beautiful form of the Mass than groups like SSPX - it makes us all look like heretics. I’d love to see Papa Francesco excommunicate them and then celebrate a licit EF Mass… probably hoping for too much there though.
As Father Z would say, Pope Benedict
still was the pope of Christian unity!