...Unless I am grounded in faith, how can I make others firm in faith? It is certain that faith belongs especially to my office. The Lord publicly proclaimed it: ‘I’, he said, ‘have prayed for you Peter that your faith may not fail, and you, once being converted, must confirm your brothers’... For this reason the Faith of the Apostolic See has never failed even during turbulent times, but has remained whole and unharmed, so that the privilege of Peter continues to be unshaken ...
That is Pope Innocent III's teaching on the doctrine in question, citing the very Gospel text that is cited by Vatican I on the unfailing faith of the Pope. He speaks of "I". He uses the first person: "Unless I am grounded in faith". And he quotes in regard to this, his personal faith, the words of the Lord, in relation to the unfailing "Faith of the Apostolic See".
I don't care what umpteen theologians think about what Vatican I meant. I especially don't care what two lay theologians who only appeal to fallible authority to make their "arguments" think (that's called a logical fallacy where I come from). It wouldn't even matter if Robert Bellarmine, Francisco Suarez, Domingo Soto, John of St. Thomas, Cajetan etc., weren't of the opinion that it's impossible for a Pope to become a heretic (they were, despite what some want you think they thought), because the teaching of Pope Innocent III trumps all of these at the ontological level.