I'm curious, why do you think they crossed the line? What line do you think was crossed?
"We are perfect; all that is needed for reunion is for the other lost sheep to return to us, groveling. Wipe your shoes before coming in the door."
That of course is your characterization, a very negative one. This is Church teaching:
"The unity of Christians cannot otherwise be obtained than by securing the return of the separated to the one true Church of Christ from which they once unhappily withdrew. To the one true Church of Christ, we say, that stands forth before all and that by the will of its Founder will remain forever the same as when He Himself established it for the salvation of all mankind." (Pope Pius XI, Mortalium Animos)
there *has* been progress in that regard (and with the Lutherans) - just not quite as much as was hoped for.
What do you mean by progress? Where are we progressing to, what goal?
The line which rejects ecumenical conversation altogether.
It seems clear there is no going back on that one.
My gloss of the "Fortress" may seem polemical, but I get a whiff of it in this letter and stronger ones in some other traditionalist communities, where any efforts by Rome to secure any reunifications are looked upon with suspicion if not outright dismay. I don't think anything Pius XI said here contradicts such dialogue.
By "progress" I mean ut unum sint that they may all be one - that we may all be one, that the Body of Christ no longer be divided. I certainly do not favor "unity" over Truth (for such would be a false unity). But even with a true unity, this seems a highly unlikely prospect for the foreseeable future, until the Holy Spirit decides otherwise; but for my part, I would very, very much like to see the Church "breathe with both lungs again." Despite Russian intransigence, I think that full communion with the main Eastern Churches is within reach without sacrificing anything in Church doctrine.
But someone has to do the hard work of resolving the complex difficulties of the Filioque and the ecclesiological questions. Simply saying the door is open is not going to achieve anything.