Regular NDAs I have signed are bi-directional, they protect both the interviewer and the interviewee. E.g. if you interviewed and decided not to take the job but the interviewer called your boss and told him that you're looking to leave your job with a goal to get you fired then you could probably use the NDA to sue. The NDA the recruiter offered was not one of those, it was protecting only Apple so I don't know if they have the same for on-site interviews or more regular one.