What we have here is the ecclesiology of Imperfect/Partial communion. Is this an error? Certainly it does not sound like anything ever openly taught by the Magisterium or by Catholic theologians.
For those who think that this teaching is erroneous, answer the following question:
Can an Orthodox "Christian" who errs in good faith be saved?
99.999% of Traditional Catholics will answer, "Yes, such a one can be saved."
Well, OK, then.
But there can be no salvation outside the Catholic Church.
If this Orthodox person is to be saved, then that must mean that he is inside the Church somehow.
So one way to look at this would be to say that the person is formally but not materially a Catholic.
So, Catholic in one sense but non-Catholic in another sense.
So, by virtue of being Catholic, one of our "brethren", but by virtue of the material separation, "separated brethren".
So, by virtue of being Catholic, in communion, but by virtue of the material separation, the obstacles that divide us, not in "full" communion.
But matters of "good faith" and "sincerity", etc. belong to the internal forum, and not even the Church declares Herself competent to judge the internal forum (de internis ecclesia non judicat).
In the past, before Vatican II, the Church simply made a PRESUMPTION, then, in the external forum, that those materially separated from the Church were in fact not within the Church.
After Vatican II, in the interests of better relations with the "separated brethren", with the desire and intent of bringing them closer to the "fullness of truth" in the Catholic Church, so, for pastoral reasons, the Church now makes a PRESUMPTION in the external forum, of "good faith" on the part of the separated brethren due to their having grown up in these non-Catholic groups and so therefore not being guilty of the actual "sin of separation".
Is this, then, anything more than the spirit of being "positive" that "Good Pope John" wanted to promote? Is this, then, anything more than taking a "glass half full" rather than "glass half empty" perspective upon the same reality? In the past, our PRESUMPTION was pessimistic; now our PRESUMPTION should be optimistic. In both cases it's just a presumption.
In many ways the Church operates on presumption with regard to the external/internal forum problem. So, for instance, the Church gives Holy Communion to many, many people who are not in a state of grace, and so participates in sacrilege, based on the PRESUMPTION that they ARE in a state of grace. So, then, what's so different about offering Holy Communion (communicatio in sacris) to Orthodox based on the PRESUMPTION that they are in good faith (similar to the person in mortal sin kneeling at the communion rail)?