not only it provides damage bonus vs fiends, it also provides an additional low defense against fiends. this is shown while wearing a fallen whilst having a valkyrie or divine veil. the fallen set usually gives out medium minus damage and defense vs fiend, valkyrie and divine veil cancels out the damage penalty, and makes the defense penalty low instead of medium.
this could probably be true to those family damage bonus armors (e.g. chroma line), but there are no armors that have negative resistance to support it. i would wish that OOO would reveal this in the wiki.
Do you understand what a wiki is?