Just because it's not complete doesn't make it a game 'engine' (though there's likely an engine in the code somewhere). It's more than a game engine, it's a game—it's just has no art assets. So, no, you can't make a full release from this repo, but you can build the executable.
This is also how the Doom 1, 2, and 3 sources were released. You get the code but you need to buy the game to legally get the other assets.
> Here is everything you need to build Amnesia: The Dark Descent.
But as you said, the assets seem to be missing. Maybe they release them somewhere else?