Wrong, again
After the reading of the sentence of excommunication came a long pause, for a condemned person was not denied time to address the people if wishing to do so. For half an hour or more Joan spoke, protesting her faith and trust in God, asking for the prayers of the people as well as for the intercession of the saints, and her words, "pitiful, devout and Catholic", were so moving that those who could hear her, even the Cardinal of England and many Englishmen, were seen to weep.
The soldiers grew impatient. Two sergeants came and forced her down from the platform where she stood and led her to the Bailiff who represented the English authorities. So far she had been excommunicated but not sentenced to death: yet no judgment was read in the name of the king, no sentence was pronounced, and the Bailiff, merely waving his hand, to signify these legal formalities were not worth troubling about, said: ""—that is: "Take her away. Take her away"—and she was straightway taken to the stake and handed to the executioner. She asked for a cross and a soldier hastily made one with two pieces of wood tied together—she kissed it and put it in her bosom. Then her arms were pinioned behind her back and she was chained to the stake. At her request, Isambart, who, as well as Ladvenu, was attending her, sent for the cross of a near-by church and held it before her right to the end of her long agony. "To the end of her life", affirms Martin Ladvenu, "she maintained and asserted that her Voices came from God and that what she had done had been done by God's command. She did not believe that her Voices had deceived her, and in giving up the ghost, bending her head she uttered the name of Jesus in a voice that could be heard all over the market-place by all present, as a sign that she was fervent in the faith of God." Her heart was unconsumed. By order of Cardinal Beaufort, the ashes and all that remained of St Joan were put into a sack and thrown into the Seine "that the world might have no relic of her of whom the world was not worthy".[5]
By a contradiction which shows how little the Tribunal were convinced of the justice of their own sentence, they granted her—a declared schismatic and heretic—the privilege of Holy Communion, which all these long months had been denied her.
I will grant you that the Tribunal attempted to excommunicate her; it would seem, however, that it was invalid (and hence there was no excommunication) since it was manifestly contrary to the law. As the Catholic Encyclopedia says:
The first trial had been conducted without reference to the pope, indeed it was carried out in defiance of St. Joan's appeal to the head of the Church. Now an appellate court constituted by the pope, after long inquiry and examination of witnesses, reversed and annulled the sentence pronounced by a local tribunal under Cauchon's presidency. The illegality of the former proceedings was made clear ...
In any case, the example of St. Joan doesn't seem to prove your point. Are you arguing that since she was truly excommunicated unjustly, the excommunication had no force, and hence the same is true of Msgr. Lefebvre and his bishops? St. Thomas explains why whether or not she was excommunicated unjustly or not excommunicated due to illegality of the sentence, she would not have been damned. For he says, as I pointed out in my previous post:
In this case, if the error, on the part of the sentence, be such as to render the sentence void, this has no effect, for there is no excommunication; but if the error does not annul the sentence, this takes effect, and the person excommunicated should humbly submit (which will be credited to him as a merit), and either seek absolution from the person who has excommunicated him, or appeal to a higher judge. If, however, he were to contemn the sentence, he would "ipso facto" sin mortally. (Sup., q. 21 a. 4)
Obviously the case of St. Joan, who appealed the sentence and submitted, is far different from the case of Msgr. Lefebvre and his bishops, who did and do in fact contemn the sentence against them. Hence no comparison is possible, it would seem.