Pope Celestine's letter regarding Nestorius completely backs up sedeprivationism.
St. Robert Bellarmine, quoting Pope St. Celestine:
Pope Celestine I, in an epistle to John of Antioch, which is contained in Volume One of the Council of Ephesus, ch. 19, says: “If anyone who was either excommunicated or exiled by Bishop Nestorius, or any that followed him, from such a time as he began to preach such things, whether they be from the dignity of a bishop or clergy, it is manifest that he has endured and endures in our communion, nor do we judge him outside, because he could not remove anyone by a sentence, who himself had already shown that he must be removed.” And in a letter to the clergy of Constantinople: “The Authority of our See has sanctioned, that the bishop, cleric or Christian by simple profession who had been deposed or excommunicated by Nestorius or his followers, after the latter began to preach heresy, shall not be considered deposed or excommunicated. For he who had defected from the faith with such preaching, cannot depose or remove anyone whatsoever.”
This pretty much closes the book in Siscoe and Salsa.
He "who had defected from the faith with such preaching (of heresy) CANNOT depose or remove ANYONE WHATSOEVER". So "from such a time as he began to preach such things", Nestorius had no power of office, and could not remove anyone whatsoever ... whether justly or unjustly. Not from the moment he was condemned by the Church in 431, but from the time he began preaching his heresy, in 428.
Pope Celestine designates such as one, who has defected from the faith publicly (e.g. by his preaching of heresy) as
removendi ... to be removed. So they have lost their authority, their ability to exercise their office,
formally, but are still in the condition of needing to be removed,
materially.
Cardinal Billot goes on to quote even MORE from Pope St. Celestine:
[Pope St. Celestine:]“Nevertheless, lest the opinion of one who had already called down upon himself a divine judicial sentence seem valid even at the time, the authority of our See has decreed that, from the moment that Nestorius and those like him begin to proclaim such [heresy], We do not regard as exiled or excommunicated any of the bishops or clerics or Christians by any profession who were dispossessed of office or cast out of communion by him and his followers. Rather all were and still remain in communion with Us, because a person who erroneously preached such [heresy] could not eject or remove anyone”. [Cardinal Billot]: Therefore you see that a bishop who is a heretic in secret is still vested with the power of binding and loosing, since he loses episcopal jurisdiction and the power of excommunication only from the time at which he begins to preach heresy openly.
According to Pope St. Celestine, "from the moment that Nestorius and those like him BEGIN TO PROCLAIM" heresy, their opinion is invalid since they had "already called down upon [themselves] a divine judicial sentence". Such as these COULD NOT eject or remove anyone.
So, according to Cardinal Billot: "from the time at which he begins to preach heresy openly .... [a bishop] loses EPISCOPAL JURISDICTION and the POWER of excommunication."
God takes away the ability to exercise their office from the moment they begin to publicly manifest their herersy, and then the Church strips them materially of their office later.
Let's translate this to the case of Archbishop Lefebvre and the bishops he consecrated. Since the Vatican II papal claimants have already been condemned by a divine judgment, they had no authority whatsoever to excommunicate those Traditional bishops.