can someone explain to me, is this really official ?

It's official as in it was published commercially under the SSSS.Gridman IP. It's not canon as in it's actually a part of the main story line. The Heroine Archive is a book with story recaps, staff interviews and guest artists to draw comics/illustrations.

Some short clips of Akane and Rikka having a sleep over. There's a good deal of hand holding, hugs and other cute stuff. Plus, Akane and Rikka have top tier voice actors.
https://www.youtube.com/watch?v=XQZ839Lx3aQ&list=PLaDMvVOkLFgWI0YnPBa5vT1zS5lNN-EwF

Or you can download them here https://nyaa.si/view/1146970

I liked the one where Akane couldn't insult Rikka or be mean to her. That was just too cute.

Beautiful.