My best guess it's that it's some weird limitation of the questing system that underlies the game; like it can either check if a specific item is equipped or a certain type of item is equipped, but not if both parties are wearing the exact same matching item. Rather than rework the underlying code, they figured they'd put a warning on the store so that people wouldn't be encouraged to do it (and unfortunately the original wording of that warning was a bit confusing in English; the new one is better but still not as clear as it could be).
