Originally, the Minchas Chinuch says that Bnai Ketura are exempt from giving Mila to their Avadim. They cannot be compared to Jews that actually owns the actual body of the Eved. In Gitten 38a it states that Goyim do not have the same ownership on Avadim as Jews. Though they're entitled to all work that the Eved produces, they don't own the actual body of the Eved. Since the body is not theirs, they don't need to make Mila on it.
But it's not that simple.