According to Mr. Smith in "Sky", the Metalkind are boron-based lifeforms, not cyborgs like the cybermen. Also, in the rather ill-tempered conversation involving Sarah Jane, Miss Myers (however she spells it) and the captured Metalkind, the Metalkind spoke of the Fleshkind having mined metals from Metalkind's planet to make machines and said, "They stole our children." Although the Metalkind are obviously overreacting by seeking vengeance on all flesh (i.e., carbon-based) peoples, it's entirely possible that the Fleshkind committed the first, very serious, wrong. If the Fleshkind know this but want to deny it (to themselves, as well as to everyone else), guilt could explain the intensity of their hatred of Metalkind. Miss Myers was definitely no good advert for the reasonableness and morality of her people.