"What you didn't experience in that world doesn't change in the real world either"
I kinda hate this kind of sentence. What exactly does he want to say?
I removed the spoiler tag, because you're quoting the chapter that this thread is about.
That translation is too abbreviated/unclear. What he means is that Niito/Elrise CAN watch the videos and read about the other routes (e.g. Eterna) and see how they turn out; but Niito/Elrise can't watch videos or read about Elrise's route (which is the route that Elrise is living through (much to her consternation). I.e. Niito/Elrise isn't experiencing Eterna's route, and so Eterna's route in the game is unchanged.
EXCEPT (and this is confusing): Everything in the game has changed from what it was originally. In short:
- At the start, Niito has played through the entire game and all of its routes, and Elrise is the fat villain in all of them.
- Then Niito reincarnates as Elrise and changes things.
- Then N/E returns to her original world/self and sees the game, and everything has changed...
- Elrise is no longer the fat villain in ANY of the routes. Instead she is "not a saint, but with great power, and ready to hand over the role of Saint to Eterna".
- However, (and what was not clear until now), while this Niito is able to view the other routes in the current game, he (and the visiting Elrise) can't view Elrise's route.
So... the causality of what is happening is rather confusing. The author will have a character theorize what's happening -- I think that's a little in the future of the manga. (IMO, the explanation is a bit strained. I would give a different one.)