This news comes on the heels of what is reputedly a world first Death Knight obtaining the legendary sword. Here's the kicker, though... the left and right Bindings of the Windseeker have class restrictions (revised to include Death Knights)... and Warlocks aren't among those eligible to start the quest chain. Mages and Warlocks are both casters who can equip swords, but this doesn't explain how the Warlock obtained the item. [EDIT: Whoops. Looks like I dropped the ball on this one. Daniel already wrote that even though the Bindings have class restrictions on the tooltip, they're actually usable by any class according to Blizzard.]
The Warlock's guild seems to be a rather accomplished one (the 'lock has The Immortal title), so there's a good chance that getting the weapon was a guild effort. It's quite possible that the Dormant Wind Kissed Blade that drops off Thunderaan was assigned by Master Looter to the Warlock, who then started the quest.