"Kolokotronis, is this the basis of what the Orthodox Churches teach on the validity of the Catholic Eucharist?"
I don't think so. What Met. John is speaking about is that the fullness of The Church is to be found in a single diocese. But note how this true theogoical genius, a man of great personal piety and spirituality, ties this to the universal Church and primacy within that universal Church. This is in accord with the teachings of +Ignatius of Antioch on both The Church and The Eucharist. Orthodoxy's acceptance of the validity of the Eucharist in the Western Church derives from the validity of its orders and a very long a detailed history in various councils about how to deal with sacraments of "schismatics".
As for me, well I never doubted the validity of a Latin Rite Mass, (or any Latin Rite sacrament) even before I knew anything about theology.
Met. John seems to be extracting the Orthodox notion of the primacy of the Patriarch as first bishop among equals and the primacy of the head of the Council as his basis for understanding the primacy of the bishop of Rome. I think that this is a brilliant bridge of understanding between Rome and Orthodoxy. I am very interested to see what may come of it.
Blessings to Met. John!