Is not a spec and I dont know where you got the information that he is canonically unholy when hes depicted almost everywhere as a Blood Dk. In ICC he holds a 2h weapon as well. Lore characters though do not fall under the constrictions that players do because gameplay is a thing only for players. Even in Lunarfall he has a 2h weapon.
Diseases do very little in terms of overall dps in wrath. DW also doesnt help that damage and Blood-caked blade isnt much damage to focus your build around as DW either.
Unholy in cata had 1 build for DW and it was killed. So you were just hurting yourself and the group you were in, which is fine if everyone is fine with you doing it.
In wrath you still have a lot of damage come from autos and strikes as unholy. Scourge Strike scales with weapon damage and its shadow portion scales off damage, diseases cant crit either. Diseases are just a means to getting good damage out of your scourge strike, blood strike, better damage out of your blood-caked blade and you dont have to put your diseases up outside of your first application per fight due to Glyph of Disease. Its a DW spec. You can use DW with it but that doesnt mean you should. Ive seen some DW blood and unholy players and they were slow at killing anything.
DW is a frost focused thing, 2h for both Blood and Unholy.