I read the IH story, and it turns out the cover has no relation to the story. It's more about Ferrus Mannus getting a glimpse of his legion's future, and how disdainful he is of it. He would be repulsed and enraged at their rejection of their humanity.
Oh it's like the small tidbit from Wrath of Iron where it is said that Ferrus Manus saw the obsession with machine-parts on equal ground like the flesh-change or any of the other genetical sicknesses the other Legions might have. I don't have the exact quote on me.