Less concerned about fur effects and more concerned about Mantle's chance of been used. Its a (potentially) huge AAA title and would be a blow to Mantle getting a solid foothold.
For the physx stuff I can't imagine why nvidia is ever going to let anything like that run on AMD GPU's (not sure how this would work?). Its one of their selling points and off the back of The Witcher 3 they could probably make a killing.
Better still if the game is late 2014 and mid/high end Maxwell gets a release imagine how many units they could sell if they made a deal and had The Witcher 3 bundles!
Wont take long for the community to hack it in for AMD users.
Communities hate it when companies create barriers for money.