I know all about the heart of darkness and its properties, have played BO1 also like twenty times through. That's not what he's talking about. He's looking at the design of Kain's armor and sees the chain mail gut area as "exposed" compared to the iron plate armor on his chest and shoulders, etc.
Edit: Oh, you meant my retort to Nessdark. I was joking. I know he was talking about Kain being resurrected by the HoD. I just felt like adding that