A long time ago +Sanborn's private judgement, almost overnight morphed into dogmatic fact. You should have no scruples about this issue as long as you remember that the una cuм issue (and pretty much everything that has to do with it) is nothing more than people's opinion, it's nothing more than private judgement, a theory, a conspiracy theory, that the conciliar popes are not popes.
So, the mistake they make is this, and others like, say, Father Jenkins has pointed this out, at a higher level, but does not break it down into the form of a syllogism (which is how I tend to see the world) --
Bishop Sanborn rightly identifies a dogmatic PREMISE of the implied syllogism leading to the SV conclusion. He therefore classifies all those who reject his conclusion as heretics, because, the only way to reject a conclusion is to reject, what? ... ALL the premises? Bishop Sanborn ignores the MINOR premises, that even if they're certain, even morally certain ... they are NOT
de fide, and therefore, according to the "weakest link" principle of logic,
peiorem partem sequitur conclusio, the conclusions cannot be dogmatic either. One could deny the conclusion by 1) denying a non-dogmatic premise (rejecting the syllogism materially), or 2) pointing out some issues with the formal logic (a missed distinction being most common), or a combination of both. That's actually what +Lefebvre did, hold back on affirming the MINOR premise, while explicitly affirming the MAJOR that Bishop Sanborn (in his muddled binary thinking) insists must be denied in order to reject the SV conclusions. That is false, and that thinking speaks to the same black-and-white binary thinking they apply to the "una cuм" issue, where a priest who puts the name of the Conciliar Pope in the Canon because he THINKS the man is the Pope (having made an error), or just that he MIGHT be, or even that he likely is NOT, but adheres to the Cajetan / John of St. Thomas opinion, or just isn't sure and wants to give "benefit of the doubt" ... is exactly the same as someone who deliberately excludes the Pope due to rejection of the papacy (Orthodox). For them, there's no difference between a Father Chazal, who inserts the name because he's the "visible sign of unity" (little different from sedeprivationists in this regard), and a Greek Orthodox who rejects the papacy in principle, i.e. between a materially-erroneous motive for inserting the name and a formally-erroneous motive.
Now, if +Sanborn at el. wanted to explain how or why, for these purposes, that distinction is not relevant, I invite them to do so and I will listen with an open mind, but they continue to plod along on this "una cuм" issue, causing consternation among the faithful's consciences, while pretending that such distinctions don't exist, where, according to Father Deposito, a priest who THINKS Leo is or might be the Pope might as well be a Satanist offering the Mass "una cuм diabolo". What utter hogwash That
reductio ad absurdum alone suffices to debunk their reasoning.
Now, in the next step, they could argue ... well, even if the priest is mistaken, you can't go there if you don't think Leo is the Pope. That's also false, since I could go there simply for DISAGREEING with their position on "una cuм" and thinking it's not inherently sacrilegous, blasphemous, and resulting in a "schismatic Mass" to put the name in there due to material error only, i.e. if I felt that I could have in good conscience assisted at the Mass of St. Vincent Ferrer even if I thought the one of the other guys was the pope Again, I invite arguments along those lines, but they never even attempt to address these objections, but simply ignore them, pretend they don't exist, and circle the wagons behind the comfort and convenience of their black-and-white binary thinking ... having no tolerance for gray areas.