Listen carefully while I blow your minds:
Up until now, the Legion didn’t have to be from the future at all.
There have been little things in a couple of stories that indicated the connection between their home and Superman’s, like the history lesson with the Superboy robot and the historical preservation site at the Kent house, but none of them have been integral to the story, and none of them have ever revealed anything that the non-Legion characters didn’t already know. You could write functionally identical stories featuring a Legion who came from another star system, or another universe- what’s important is that they have access to Smallville and Metropolis, and Superboy and Supergirl have access to their home, but they aren’t the same place.
Then Brainiac 5 shows up. And he doesn’t just know that Supergirl exists, that she disguises herself as Linda Lee, and that she’s Superman’s secret weapon… he knows things about Superman’s life that Supergirl doesn’t know, because they haven’t happened yet.
The decision to allow plot points to be written in advance like this- to pin down Superman’s future by writing it in the Legion’s history- opens a ton of possibilities for Legion stories… but it also raises all kinds of questions about the nature of causality, and the ethics of time travel. Can the past be changed? Is it even acceptable to try? If so, what if you make things worse? If not, what about the future?
This story isn’t going anywhere near those questions.