This argument fails the common sense test. If the two were already separate, why did they schism in 1054? Yes, the rites differed slightly and there were theological differences, but if that's all it takes to be considered a separate religion, there should be a separate religion for many many different countries and times. Pretty much every case where local pagans were converted to Christianity should have their own syncretic religion, by that metric, and changing to a a version more in keeping with the parent religion over successive generations of missionary work from the Church. That kind of organic granularity of religion would be pretty cool, actually, but it would be utterly unlike how religion has always been handled in Crusader Kings and the new "dynamic religions" system thus far described for CK3 doesn't go nearly that far: it's just a "make your own heresy" decision. I don't think we can expect the game to represent the applicable sort of theological disagreement while still being in communion. If it does, great, but then why wouldn't they just say "wait for the dev diary"? Or, hell, why wouldn't they be bragging about it already? That would be a tremendous overarching redesign of the religion system which would be a flagship feature of the new game. They definitely wouldn't have bogged up a comparatively surface level feature if they had something that fundamental to disclose.
The two churches had schismed *several* times, with the schisms getting larger and harder to resolve at each stage.
The answer though is that essentially the schism had been temporarily healed, the two churches were back on board with each other, and then they schismed *again* in 1054, and that was the last point at which they were united for any reasonable length of time. De facto the churches had already separated though, even if de jure they had not.
The whole filioque thing fails the common sense test to be honest. It's largely a matter of trying to say the same thing in two different languages, but one of them having to insert a phrase to make up for an interpretation of a word, but not translating it *perfectly*.
They might have been evasive if they're not 100% certain *exactly* how they're handling it. They might not actually have a definitive "we're having the churches split" or "we're having a united church" answer at this stage.
They might have the state of de facto schism cause 867 to have already separated churches just so they don't have to have a mechanic in place to (relatively) cleanly break the larger church in two. They may have it in place so as to allow Catholicism to have Crusades early in the case that relevant triggers happen, without also bringing in the "Orthodox" world.
They might have a united church lined up for 867 but not have revealed it yet because they're not certain it'll make it to the final version, and they don't want to announce it and then have to walk it back - which could equally apply to announcing a schism and walking it back to a united church.