Pope Embraces Orthodox Church

In his quoting of14th-century Byzantine emperor, Manuel II Palaeologus the Pope is not quoting a Latin/Catholic Church father but rather a member of the old church, the Byzantine Orthodox Church. The Church of Russia rather than the Catholic Church of Rome or the church of Paris or the Holy Roman Empire. Who was the Emperor the Pope quoted. An Emperor and consequently the Head of the Orthodox Church, which the Pope convinently did not mention the latter role.