apples and oranges - the Pope needs to be at an ecumenical council just like the Pope of Alexandria, etc needs to be, but the Roman Pope does not have a veto vote to council decisions. The Council decisions are binding to the attending Pope even if he has reservations about them.
The Pope did not attend Nicaea I, Constantinople I, Ephesus, Chalcedon, Constantinople III, or Nicaea II. In all of these cases, the Pope sent legates to preside at the Council and later confirmed it by his authority. The Pope also sent legates to Ephesus II, the "robber synod", which he refused to confirm and is not considered ecumenical. It follows that the legatine authority is not sufficient to make a council ecumenical, and that the actual consent of the Pope is necessary.
Since the Pope did not attend the 880 Synod, and did not confirm it, it follows that the 880 Synod cannot be considered ecumenical.
The bishops of every nation must acknowledge him who is first among them and account him as their head, and do nothing of consequence without his consent; (Apostolic Canons, XXXIV)
Over these you ruled as a head over the members, among those holding office, displaying your good will. (Council of Chalcedon, Letter "Repletum est gaudio" to St. Leo I)