So for me it's based on the Siri Theory. I agree in principle with the formal/material distinction (makes perfect sense) ... but just believe that Siri was the rightfully-elected Pope, and that he was forced to step down under duress, which would make his resignation invalid, and thus legitimate election to the office (the material aspect) would have been impeded by Siri's rightful posession of it, until his death in 1989. By that time, there were no legitimate electors left form the Pius XII era, or if there was 1 or 2 (I doubt it due to the age limit started I think by Montini ... perhaps for this reason?), who knows who they voted for.
Others believe that Bergoglio's election was (materially) invalid due to the collusion (St. Gallen mafia) or alternatively because of his non-resignation (munus vs. ministerium).
Interestingly, just the other day on Taylor Marshall's show / video, someone chatted in (the SuperChat where you pay to get your question answered) about whether any popes in recent memory had been elected but turned down the election. Marshall casually mentioned that Siri turned it down in 1958. Oh, really, Taylor? So, how do you know this? Siri never said one way or the other, since he said he was bound by the oath. Consequently, that was clearly an attempt by Marshall to contradict (falsely, with no information) the Siri Theory.
I don't know exactly what Marshall said, but based on your explanation, I don't think he was contradicting the Siri Theory in the way you think. He was just catering (or he believes it himself) to the Indult/anti-conspiracy-theory crowd, the naive bunch of "conservative" catholics who can't grasp that there are actually evil people in the world.
These people will read about the vision of Pope Leo XIII and how God will let satan try to destroy the church.
These people will read the Fatima prophecies that the 'errors of russia' will lead to the good being martyred and millions of souls going to hell.
These people will read the stories of the infiltration of communists into the vatican starting in the 1930s or the stories of Pius IX being imprisoned in the Vatican.
And they'll think "it can't happen in my lifetime."
I have so many family members who just won't/can't accept that evil people want to do really evil things. There's a blindness there which I can't grasp. So either Marshall has this personal blindness or he's furthering it by catering to those who do. Either way, such a mindset isn't going away.
The "conspiracy tin foil hat nutters" propaganda is the greatest lie in modern times, 2nd only to "the devil doesn't exist" lie. It's an amazing piece psychological brainwashing. Hats off to our enemies on this one. You gotta give them credit.