This episode takes the story from the Future Arc, and mixes in the characters from the Despair Arc to add a finish to the Future Arc. I found this ending to add very little that wasn't obviously going to happen. On top of that, it actually tossed on more questions and confusion by showing Nanami and Kyouko in the scenes. Something about an AI or imaginary image? The story of the 2 Arcs was doing just fine without having ever played the game, but this must either be explain in one of the games or just tossed in, either way it was a poor choice when a good chunk of viewers will most likely not have played the games.
You can safely skip this episode and come up with a better conclusion in your own mind of how you wanted it to turn out. The options here were whether they are able to stop the Hope video from being broadcast or not and that is answered here, no real further value or enjoyment is to be found unfortunately.
Review of Season 1 ; Scored 9.5
Review of Despair Arc ; Scored 8
Review of Future Arc ; Scored 6
Just to clarify on the Chiaki scene you're referring to, it is 100% in reference to the 2nd game, which takes place in a virtual setting used as a rehabilitation environment for the Remnants. It actually was mentioned rather early in the future arc, but quickly took a back-seat to everything else that happened in the anime. It basically just cemented how Chiaki wound up in the 2nd game.
Nonetheless, I do agree that as a standalone episode, it really doesn't have much for people who haven't played the games. Hell, I'd consider arguing the same for the entire anime. It's basically a spectacular way of tying up loose ends and paying off the three prior games, and as a result, there's a LOT of information this anime glosses over on the assumption that the core audience is already mostly aware.