I am a sedevacantist. I know very little about Our Lady of La Salette, though I have heard these prophecies.
Actually, Our Lady of La Salette was talked more by the homeschooling mothers (mostly conservative, Novus Ordo, and Indult) in our homeschool group than in my chapel. In fact, I really can't remember the last time this was brought up by any sedevacantist I know.
I realize that Matthew has known some wacky sedevantists in his life and assumes that most of us think just like the Dimond Brothers, but I really wish he'd stop posting his opinion about what sedevacantists do, say, and believe because most of the time he's either wrong or, as my son says, over-exaggerating.
I just asked two of my adult kids what they know about Our Lady of La Salette. After thinking a moment, my daughter said..."It was an apparition of our Lady? In France?" Essentially, even my kids don't really know anything about La Salette. It's really not a factor in understanding that Bergoglio isn't Catholic.