I'm not sure if it is for the new NMs or not, I've personally tested incapacitating the right arm on Kokoroon and have not seen any significant changes as far as drop rates, etc. goes. I'm still at about 15 scale mails and only 1 bow drop. So I would say incaps don't do anything on these particular NMs, however some NM incaps do cause/increase item chances (horns on buffalo, right arm on gob for sword, etc.)