If Ecumenism claims that there is no One True Church, Catholicism does not hold to it. I have never seen that definition for it, and know it is NOT what is meant by ecumenism when the Pope says it. The Catholic Church DEFINITELY claims to be ONE, Holy, Catholic, Apostolic. We msot certainly do NOT admit other churches as true churches.
Now other churches can have varying levels of the truth. For instance, Orthodoxy would be as close to the truth as you could really be without having the WHOLE truth. Episcopalian would have less truth than Orthodox, Lutherans less than Episcopalians, and Fundamentalist Baptists less than Lutherans.
But that is NOT to say they are true Churches, merely that they have SOME of the truth in them.