Which fills all the criteria of an Ex Cathedra statement as defined by Vatican I. If this isn’t an Ex Cathedra statement, what is?
Emperor Justinian (who is a Saint in the Eastern Orthodox Churches and the Eastern Catholic Churches)
responded “I invited you to take measures in common with the other patriarchs and bishops with respect to the three chapters. You have refused this, and now wish, for yourself alone, to give a judgment in writing. But, if you have, in this, condemned the three chapters, I have no need of this new document, for I have from you many others of the same content. If, however, you have, in this document, departed from earlier declarations, you have condemned yourself.”
(Which seems to suggest Justinian had no conception whatsoever of Papal Supremacy or Papal Infallibility).
The Fifth Ecumenical Council, composed of entirely Eastern anti-Three Chapter bishops, then proceeded on to discuss about the Three Chapters:
“When, for example, Vigilius, Pope of Old Rome, came hither, he, in answer to our questions, repeatedly anathematised in writing the Three Chapters, and confirmed his steadfastness in this view by much, even by the condemnation of his deacons, Rusticus and Sebastian. We possess still his declarations in his own hand. Then he issued his Judicatum, in which he anathematised the Three Chapters, with the words, Et quoniam, etc. You know that he not only deposed Rusticus and Sebastian because they defended the Three Chapters, but also wrote to Valentinian, bishop of Scythia, and Aurelian, bishop of Arles, that nothing might be undertaken against the Judicatum. When you afterwards came hither at my invitation, letters were exchanged between you and Vigilius in order to a common assembly. But now he had altered his view, would no longer have a synod, but required that only the three patriarchs and one other bishop (in communion with the Pope and the three bishops about him) should decide the matter. In vain we sent several commands to him to take part in the synod. He rejected also our two proposals, either to call a tribunal for decision, or to hold a smaller assembly, at which, besides him and his three bishops, every other patriarch should have place and voice, with from three to five bishops of his diocese.”
“Constantine, the most glorious Quæstor, said: While I am still present at your holy council by reason of the reading of the documents which have been presented to you, I would say that the most pious Emperor has sent a minute (formam), to your Holy Synod, concerning the name of Vigilius, that it be no more inserted in the holy diptychs of the Church, on account of the impiety which he defended. Neither let it be recited by you, nor retained, either in the church of the royal city, or in other churches which are entrusted to you and to the other bishops in the State committed by God to his rule. And when you hear this minute, again you will perceive by it how much the most serene Emperor cares for the unity of the holy
churches and for the purity of the holy mysteries.”