"An Ecumenical Council requires the presence of the Pope or his legate" - there were no legates at First Constantinople. How could the Church in the West be represented without her Patriarch, anyway?
In those days, all bishops were still considered equal, so I suppose the Romnan See did not object, and the Church did not have one Patriarch.
Councils were called to convene by the Roman Emperor, and the term ecumenical in those days was synonimous with imperial.
I am not exactly clear as to why the Latins recognize Constantinople I. Maybe you can shed some light on that. But, then, you can also disavow it as ecumenical -- it's all the same to me.