"You may very well be right on this one..." Well., it looks as if he may be right as to the dating of the controversy on the azymes, but not as to the issue itself.
Someone changed the Tradition as to the matter of the eucharistic bread because it started with one type of bread, leavened or unleavened. That start, according to Canon 32 of the Council in Trullo, was the Liturgy of St. James, Protobishop of Jerusalem and Brother of the Lord. Either all of the eastern sees outside of Armenia and Rome changed the Tradition or else Rome and the Armenian sees changed the Tradition. The Tradition was one to begin with and there was teaching, on all sides, as to the reason for either azymes or leavened bread. As you correctly point out, Apollinarian Christology in Armenia was feared by the bishops who met in Trullo.
Our Lord said he would build His Church on the Rock of St. Peter the Holy Apostle, that being his Faith, and that the "gates of hell would not prevail against it." If either azymes or leavened bread are the novel practice and the other is the more ancient one, and yet both obtain in the same one Church, that's chaos and the gates of hell have prevailed.
Our Lord used unleavened bread, it being the Passover. There can be no older practice than this. Yet can it not be possible that all that the only necessity is that bread is used?