It is not necessary to write much about what follows. You know the difference between true and false ecumenism.
For those who do not, a quick refresher, it really is quite simple.
True ecumenism is to preach that all Christians ecclesial communities have a duty to return to full and complete and visible unity in the Catholic Church outside of which there is no salvation.
What we have primarily experienced since the Second Vatican Council is a false ecumenism, that we can dialogue and leave people where they are because God just loves them there, anyway. Even though, "there" is out of union with the Church and in heresy or schism.
We don't need to pick a fight on an individual basis with our Orthodox brothers and sisters. Frankly, given the behaviour of Rome and the those in the west who undermine the Church's faith, liturgy and morality, what possible reason would the Orthodox have for wanting to discuss unity.
To equate that one can "sin" against "ecumenism" is an utter falacy. It is repugnant to think that a Pope could invent a sin against an action or a theory or a movement. One sins against God, not an idea.
“Be open, be a friend. ‘But I must do everything to convert them!’ There’s a great sin against ecumenism: proselytism, they’re “our brothers and sisters.”