What about it? It is entirely supported by cuм Ex Apostolatus and St Robert Bellarmine knew it.
I agree with Bellarmine that a Pope cannot be a heretic.
Well, I agree with Bellarmine that a Pope cannot BECOME a heretic (fall into heresy after election). But that's just pious belief.
I believe that what happened here renders all this argumentation between Bellarmine and Cajetan / John of St. Thomas moot.
These guys were not legitimately elected. Cardinal Siri was elected Pope Gregory XVII (accepted and even chose a name) ... and then was illegitimately forced out of office under grave duress.