So which one is it ...
1) not pope because he wasn't validly elected?
2) not pope because of public heresy?
Those two are mutually exclusive. If he wasn't validly elected, then he couldn't be ipso facto deposed on account of heresy, since there's nothing for him to be deposed from. If he was deposed for public heresy, then that presumes he was valid in the first place.
This demonstrates again that you just don't want Jorge to be the pope, but want the pre-Bergoglians to be legitimate, and so you come up with whatever reason you want to throw at the wall to make him a non-pope.
There's no heresy other than that in Amoris Laetitia that his predecessors did not also hold and in fact originated. Period.
I am not certain that Jorge Bergoglio was a public manifest formal heretic prior to the 2013 conclave. I know that Archbishop Vigano holds that he was indeed one. However, I have not looked into this question sufficiently myself. What I am certain of, however, is that Benedict XVI did not validly renounce his office, so he remained pope until his death on December 31. 2022.