The point is that Deathwing isn’t wearing armor to blunt attacks, or to shield himself from an external threat. The plates on his body are holding him together, like a prosthesis.
Dracthyrs won’t be getting internally ripped apart by their own power, so they won’t need Deathwing-style armor plating. Dracthyr also won’t be mounts, so they won’t need that type of armor either.