I think Teach inherited his “special body” from Rocks. And that its basically a property of the Davy clan.
I say this because Rocks must have had something that made him so dangerous. He had advanced haki, but so did Roger. Why did Roger then need Garp’s help to face him?
We saw that Rocks was fighting on par with Harald, who is an ancient giant.
That is tbh crazy, because giants typically should outmatch a human in strength. Sure, a human could best a giant due to haki or DF power. But strength? That’s where giants excel. And yet Rocks clashed on par with Harald.
Rocks was basically a haki master like Shanks/Roger, but with the strength, endurance, and durability of yami-less Teach. Basically an ancient giant with the body of a man.