In the most basic sense, it is information that we know, but the characters do not. On the other hand, it's not information that affects how the characters behave. If the characters knew there was a different, modern society 1000 years ago, but then people got powers, misused them, and now things are different, would they behave any differently than they did in that episode? I doubt it. That basic tale might even be part of their literature curriculum.
I guess you're right that it's not necessary (or it doesn't seem that way) that we know this right now. Still, it seems like the idea that this world arises from our present matters to the author. Going by that interview, he cares that we believe the way these characters act, and the events that result, are an aspect of the way we live now, and not just something rooted in a fantasy universe.
If it turns into a show entirely focused on learning what happened in the distant past to the exclusion of the current setting or proper character development, then that'd be a shame. But Nier avoided that even though it started with a plot hook 1300 years prior to the rests of the game. I'm sure that this can manage the same thing.