IMO, DS interest here is not to sue for slander anyone, even if such a lawsuit would be actionable (and I think it would be) because that might involve a civil trial and he does not need the attention it would generate under this highly charged climate, not to mention the proverbial can of worms that such a lawsuit might open independently of his guilt or innocence in MP disappearance, and I think the grandparents, when making those statements, are well aware of just that.
Besides "not cooperating" can mean different things to different people and while it might even be technically true it might also be perfectly understandable as in the example I made in one of my previous post.