- Believing in chaos doesn't make it fel or void. Elementals are chaotic, yet they're not fel or void.
- Iyyokuk doesn't even SAY that warlocks are priests. Iyyokuk isn't even a priest, you're grasping at straws here.
- Shamans commune with the elemental plane, and their ancestors.
- Necromancers play with death magic, which is totally different from fel. Even the Void and Shadowlands are separate from each other. As is the Twisting Nether.
You want to see holy wrath and holy fire in action? Play a paladin. Consecration and Holy Wrath. You want holy fire to rain down? Play a disc priest, use Holy Fire.
OP is speaking of Orc PRIESTS. Not 'priestly' orcs. Not orc warlocks. Orcs as warlocks within the Army of Light doesn't even make sense. Orc priests makes more sense considering the exchanges orcs have with their ancestors and their similarities with Troll and Tauren priest and shaman cultures.
Following your line: every race that has a warlock doesn't need a priest class because warlocks are 'priestly'. This is an absolutely false premise, proven by gameplay and lore. A dislike of Chris Metzen or any writer won't de-canonize what has already been established and clarified. You can't play a warlock and pretend you're a priest, and vice versa. Hell, they gave priests the Lovecraftian theme.
Going back to OPs point. Orc priests are possible, considering the class fantasy behind priests, and the involvement orcs have with the Twilight's Hammer and the Shadowmoon clan. The Army of Light would shed information about the orcs' ancestry.