OK, so it happened that Pius XII just got tongue-tied and got two sentences mixed up in his head and inadvertently asserted the "not". So clearly manifest MATERIAL heresy no longer suffices to effect deposition. You also correctly anticipated the next step by saying that he has to be given the opportunity to recant.
So we moved very quickly from MANIFEST HERESY deposes ipso facto to MANIFEST FORMAL HERESY. We've already added another adjective / qualifier.
It indeed is as simple as he has to be a public heretic. A slip of the tongue does not a heretic make.
Oh, but it does. It make a material heretic. Which is why I had to add the additional qualification "FORMAL" to heretic.
So let's get back to a point that I glossed over, since it doesn't appear anymore that you've conceded it.
Let's say that Pius XII said something that wasn't a slip of the tongue but he didn't know was heretical. Someone points it out to him, and he immediately recants once it was pointed out to him. Did he fall from the papacy? Did he fall from the papacy and regain it upon recanting? Did he not fall from the papacy at all since he was not obstinate and therefore not a formal heretic? Did he fall from the papacy and now a new election needs to be held?
I'm sure that you'll concede that a statement made in ignorance does not cause one to fall from the Church because it's not FORMAL heresy.