My own thoughts on what these screenshots mean: I think it's a dlc pack. I do think an engine upgrade might be included. I'm a bit foolish in this nature, haha.
I don't think you could call it Legend of Grimrock 2 because of the ending of Legend of Grimrock.

This is the way I hope the dlc will work.
It will be released pay to play, but with the assets released like the asset pack. This way, any modders who use assets from the dlc won't be contributing to their mod unless they modify the files.
Which, in seeing the new tasty wallsets, would save a ton of space, mod-wise.