I'm not familiar with the JoJ entries, but the lore quote I posted only says that the armor was used by the trolls, not that it was made by them.
The Darkin, to the best speculation I've seen, are weapons themselves that possess their users. Thus, Varus is most likely in the process of being controlled by the Darkin he wields. Aatrox is "the Darkin Blade", not the being that wields the blade. In this same train of thought, Warmog's may have possessed it's users for a while. What's unknown is why Warmog's would reject it's wearer and kill them.