Source: http://www.vg247.com/2013/04/04/thief-dev-wants-the-challenge-to-be-in-game-not-in-the-controls/
Pretty clear indication that the game is from casual players to casual players. While I'm not very intersted on the game myself, I still hope it sells nicely, because I sure wouldn't mind another Deus Ex game.
Amazing what the definition of hardcore is now, at least from that quote.