I found a comment somewhere else in Wowhead that says:
This particular set does NOT show up under the Set Appearances tab. You will get this set from winning a Warfront. Do not get this set confused with the set you see listed under “Warfront” in the appearances tab as this set does share the same name. The set that shows up under “Warfront” in the appearance tab can only be completed by getting pieces of gear off of the rare mobs out in Arathi, doing World Quests in Arathi, and Emissaries.
Maizou explains very well how it works: