"…simply cannot be true…"—Because you say so?
At the risk of repeating myself—He can be elected (Vacantis Apostolicae Sedis §34) and then fall "automatically "immediately" "without need for any further declaration" (cuм Ex Apostolatus Officio).
"Deciding his status is a new idea…"—There is nothing new about recognizing manifest heresy and manifest heretics.
No, not because I say so, because as I said, it is in contradiction with the law because as the pope, there is no provision in the law for self deposition, which means he either has to retire or die to get out of it.
Ok, for the sake of argument I agree, he can fall "automatically and immediately." Now, lest the Church be further divided, by what official proclamation does the whole Church know that he a) self deposed, b) we are not to listen to him, and c) he is no longer pope?
No, there is nothing new about recognizing manifest heresy and manifest heretics, the Church in fact has always obligated the faithful to do this. Deciding that the Seat is Vacant while a living pope occupies it is what is new, this, per the Canon of St. Vincent of Lerins, is what is not Catholic - because it is an idea that has never been held as a part of any Catholic doctrine through all the generations of the Church by the vast majority of the people.
Now you will want to repeat that his heresies decides he vacated the seat all by himself, the thing you are not accepting is, being that is impossible to prove without an official declaration means that that is an idea which is really only an opinion and nothing more.