Dustbringer can only be obtained through fishing. This is an homage to player speculation (from many moons ago) that Ashbringer could be fished up.
Nat Pagle allegedly discovered the location of Ashbringer, and wrote it in Nat Pagle's Guide to Extreme Anglin', of which players only have the last page. Nat Pagle is a fisherman, so players thought that Ashbringer could be obtained via fishing.
The full copy of Pagle's book, A Thoroughly Read Copy of "Nat Pagle's Extreme' Anglin", can be found in Dire Maul but is unreadable. Also in Dire Maul are the Shen'dralar, who will tell you that Prince Tortheldrin will reveal the location of Ashbringer if you bring him the Head of Nefarian (this is not true). You can surmise that the Prince read Pagle's book and discarded it in Dire Maul where we now find it. This was probably part of a quest line to obtain Ashbringer that was scrapped between when Dire Maul and Blackwing Lair were implemented. The Shen'dralar may have been the intended reward faction for Blackwing Lair but were scrapped as well, which may explain why they have no rewards. The whole Ashbringer story has been retconned as well (it was never lost to begin with).
Blizzard kept their mouths shut on whether players could actually obtain Ashbringer or not for more than a year. Some players continued to believe that it could be fished until it was finally revealed that it's not obtainable... yet.