I feel like we're gonna need to dig out the citations for expansion timelines, because suddenly we have three different claims for when Endwalker took the form we know; at Shadowbringers' launch (so, July 2019), two or three months after ShB's launch (so, September or October 2019), or the pandemic (so, March 2020 at earliest). Those first two mean that the dream couldn't be foreshadowing for Theoretical All-Garlemald Expansion, because it was already off the table by then (5.2 releasing mid-Feb 2020, and patch story being easier to edit and change before release).
And I can safely hurl the third one out the window, because my source was the big Famitsu interview. Translations over here, but the relevant part:
So yes, the deadline was brought into being by the CGI studio needing the outline that early. The story retreat over 'autumn and winter of that year' (so at some point in September to December) was when it took form. So it seems like the general guess of 'September-ish' for when the structure was nailed down is accurate.Interviewer: When did you decide to bring the story so far to a conclusion?
Ishikawa: After Shadowbringers released, around the end of 2019, Image Studio [note: the CGI studio within Square Enix that produces the high-quality cinematics in various SE game] had to be provided with storyboards. In autumn and winter of that year, we went on a residential together in order to decide the main points of the story for the next expansion. But the problem was… we’re used to the writing process, and so feeling that “here and here are the remaining parts of the story we have to talk about, so let’s accept the rest”, the residential went very smoothly. I was in charge of filling in the details of the story elements, but when I looked at the structure of the content again, I thought “this might be okay for just another expansion, but if this is labelled the ‘finale’ is it really okay?” So I reworked the story from the plot outline. The end result was ready 2 days before the deadline.
Which means by the time of 5.2's writing and scripting in early 2020, Theoretical All-Garlemald Expansion was off the board, meaning that it couldn't have been an intended plot thread for that. Again, there was still probably room for a lot of rewrites that meant things got cut, but it was cut from the structure of Endwalker as we know it, not from an entirely theoretical expansion that you can build up to be anything you want because of its absence.
Incidentally, another question in that interview confirmed that when Theoretical All-Garlemald Expansion was in the cards, the idea was that Anima would be the final boss, which gives us the general idea of where exactly its part of the story would've ended. ...but that also confirms that Anima would've existed, meaning that it still would've had the Telophoroi and the towers.