cuм Ex is simply codifying into the Church's positive law what all the Fathers and Doctors taught: if you're a heretic, you can't hold office in the Church, full stop. It doesn't matter if you're validly consecrated, it doesn't matter if your appointment is accepted, it doesn't matter even if you're elected pope and all the cardinals think you are pope. If you're a heretic you can't have an office, full stop.
cuм Ex is from the 1500s. Modernism being an 1800s (til now) attack on the Church, it seems apparent that St Pius X and Pius XII's changes to the rules for papal elections were meant to avoid a situation where most of the clergy was affected with modernistic heresy, therefore the papacy would end,because no one would be eligible to be elected. (The end of the papacy, of course, is one of freemasonic's goals, which St Pius X was WELL AWARE OF). So Pius XII re-affirmed St Pius X's conclave election rules, which suspend excommunication (and all other) penalties, so that the papacy could continue, even if it was held by heretic. This is clearly the intention of the Holy Fathers - to suspend cuм Ex for a very specific reason, after which election, the penalties return.
It is certainly a support to the idea of sedeprivationism, or a spiritually "empty" office of the pope. Yet, I think we still would need a Church decision that "pope A was excommunicated/a heretic" before we could say that the spiritual office was vacant. So it would be a case of "future proves the past", where a pope was declared an anti-pope by a future papal administration.
34. No Cardinal, by pretext or reason of any excommunication, suspension, in-terdict or other ecclesiastical impediment whatsoever can be excluded in any way from the active and passive election of the Supreme Pontiff. Moreover, we suspend such censures for the effect only of this election, even though they shall remain otherwise in force.” (Cons. “Vacantis Apostolicae Sedis,” 8 December 1945)