Elemental Wands/staffs

Any element not used by a job is considered opposing.

Not by definition, no. That's something you made up by yourself. We don't even have opposing elements in MS since enemies are not of an element, just weak, neutral or strong to them. Opposing elements in MS could be fire-ice, holy-dark and lightning-poison even if the last one makes little sense. Not to mention dark element goes by unaffected by either boost or reduction.