That ok. Is great that they open sourced their engine, however they don't even mention the assets and that you need to buy their game to have them, which is a bit strange. And they even say here is everything you need for the whole game.
When it comes to open sourcing game code, it is pretty standard.
I just think this is misleading and probably just a marketing act for the next release of their game.
Personally I find it great that they released their engine again, the more open source code the better, however they should have just been more careful with their press release. Some reporters that don't look to close to the code might think that Amnesia is now a open source game like 0AD or recently unvanquished, that actually tries to make their assets fully open source compliant.
It's hard to interpret that phrase but it certainly seems to apply to their art assets. You could ship a GPL reimplementation or total conversion mod of their game tomorrow and use the game assets, as long as the user already owns the game. Just like all the very popular Quake and Doom stuff, for starters.
You just won't get the free game that you want.
Building -> a functional exe is produced.
Playing -> Many EXEs expect a set of resource files to be in the directory to further their execution.
They don't actually state that you have to own the original game on their website.