The fluff for the Tyrant Guard says that there are reports (heretical lies of course

) of them possessing the Space Marines trademark Fussed rib cage and Black Carapace, due to their design aiming to be the ultimate form of resilience, so they can take the hits for the Tyrant.
Now the fused ribcage I get, sounds mighty useful. However, why do they have something akin to a Black Carapace? As I understand it, the Black Carapace is a kind of plastic layer inserted under the skin, which sends artificial nerves into the body, allowing the Space Marine to interface with his armour. So why do the Tyrant Guard have it?