Alexander VI was not declared an antipope later. He is listed as a pope, just like St Peter and St Pius X. Why should he not be? What was the irregularity in his election? As for "losing" the papacy, what heresy or error did he hold or teach?
The Dominican firebrand sure sounds llike he was being literal. I think he was. I also think that certain Catholic authors who rightly have defended Savonarola against charges of having laid an egg that Luther hatched were employing some sort of mental reservation when they made him out to be a paragon of Catholic orthodoxy. The truth is more complicated than that. Trickier. More embarrassing. More troubling. Less compromising to the authorities who brought about his death. In a way.
His politics were revolutionary. "The People of Florence under Christ the King!" Where did that leave the de Medici and what ought to have been rendered unto them? And this blast against His Holiness the Lord Pope seems to lack a certain Scholastic rigor, to say the least.
There is a venerable Catholic tradition of angst about Savonarola. St Philip Neri venerated him as a Saint. Our pastor, a learned man, told us that this displeased Cardinal Ghislieri, who later became Pope St Pius V. (I've never found a source for that). But St Pius was not one to tickle the Swiss Guard either. He was not a strong spiritual individualist or an original thinker. Some Saints are. (Francis of Assisi, Therese, Teresa, John Bosco, Augustine, Paul, the Queen of Saints, Philip Neri.) Some Saints are not. (John Eudes, John Francis Regis, Casper Bufalo, Mary Mazzarella, Bernadette, Isaac Jogues, John Berchmans, James the Less, Zachary, Joseph, Pius X.)
Asked what he would do first thing when he got to heaven, Pope Pius VII answered, "Get to the truth about the Immaculate Conception, and then about Savonarola."
That is an amazing statement, if you think about it.
Catholics have countless Saints to inspire and edify them. But only one ghost to haunt them.
As for His Holiness Pope Alexander, Blessed Osanna of Mantua was praying for him one day. Christ appeared to her. He looked so angry at her for praying for such a man that she stopped.