I have NO idea what this is. Geoff said on-stage it’s a co-op action adventure game, so this doesn’t seem like a sequel, but more like a standalone DLC? This re-uses a lot of enemies and assets from the original, and even Nameless King from DS3 is here.
EDIT: It’s a co-op roguelike that can be played singleplayer. Very weird! https://www.ign.com/articles/elden-ring-nightreign-we-played-it
What they said is true. At the time when they made that statement, this was already done 😎