Just that in general he's not to be trusted.
I'd like to know where he says he disposed the body. The easiest way, living in an island, would be in the ocean if he had access to a boat. But, say he didn't have one, without witnesses becoming involved, he could have buried the body in a spot that the FBI and police still haven't looked. And several years from now he'll try to make yet another deal to be moved to a cushier prison in his aging years, and say he could lead Natalie's mother to where the remains have been buried all this time. Partly to keep an ace in the hole if he needs one, and just to cause her more trauma yet again. She'd finally have Natalie's remains, but the years of trauma and torture he puts her through is something I see he can do.