I follow Regan's chess work pretty closely and I don't know where Cowen got this term. The link provided shows intrinsic ratings, not "nettlesomeness". As shown by Regan and others, Carlsen simply plays more accurately than anyone else. Chess is probably drawn, so one only wins when his opponent makes mistakes.

Both Regan and Guid & Bratko have tried weighting the accuracy of moves by the complexity of positions faced. Carlsen is middle-of-the-pack in terms of reaching complex positions, so this doesn't seem to be the mysterious "nettlesomeness" either.