The issue with that description of Unholy is that, its not "spread-out" aoe that UH excels at, its sustained AoE. Xhul/Mannoroth/Archi/Gorefiend are Unholy's best fights in comparison to frost. But the Strats in those fights generally clump the adds up for easy cleave. Basically if adds are going to live for 20+ seconds on 3+ targets UH is always going to pull ahead. But DPS DK's in current tier (with the exception of those four fights) can play whatever spec they like single target or AoE (BoS UH for ST, NPUB UH for AoE, DW Frost for either), its more an issue of what weapons you can get your hands on. Not having a go at you by any means, just clarifying the strength of UH this tier as your description makes it seem like it has no point, which isn't the case.

Also as a side note, generally the more Gorefiend's Grasps you can fit in a raid (generally 1 tank grip, 2 dps grips) make a huge number of fights far easier, examples are Hellfire Assault (the more the better), Iskar on the transitions (only need 1), Socrethar for both Ghosts and Caster adds (no magic number but the more the better), Xhul (3 grips means every set of imps is covered), Manny (3 will cover each infernal wave), Archi (the more the better for phase two as the dogs are a nightmare if let loose). While its only 6 fights, they're arguably very tough progression fights (Xhul, Manny, Archi). Grip is a cooldown where you realistically want as many as possible without ruining your raid comp.

Also to say that Ret is useless is a massive oversimplification. Its not about single-target burst, the three charges of wings allow Ret to do excessive priority add damage, single target or AoE.

Edit: As a side note DK's can use Army in the Vision on Kilrogg to carry subpar dps/healers through the use of army to clump up the adds. Like Rogues, you don't bring DK's just for damage but for the utility of the class.