And yet, couldn't the same criticisms be thrown at several major plot points from the main game itself?
After all, we don't know if the Maker exists, how Corypheus managed to assemble the Venatori in secret, how he allied with Nightmare, what happened in Redcliffe if you sided with the Templars, what the Elven Gods actually were if not "Gods", how Mythal was murdered in antiquity or what betrayal she suffered that she and Flemeth are so eager to avenge, or the true answer of what exactly was the figure masquerading as the Divine in the Fade?
There was a lot of vagueness in the main game, some of it I'll grant should have been explained better (Redcliffe, the Venatori's formation and Nightmare), but the others are minor enough that not having an answer doesn't really detract from anything.
Dragon Age has never gone for giving us all the answers right away, they've purposefully left certain things a mystery or revealing new information that flips what we thought we knew completely on it's head.
We spent three games believing that the Elves were innocent victims in the downfall of Elvhenan and the Dales, rather than having had instigated some of their own misfortune due to their society being just as bad as Tevinter and Orlais. It also took us three games to know just what Flemeth actually was, while the true origins of both the Blight and the origins of the Darkspawn have still yet to be revealed, as the most we've gleaned is that the Golden City was apparently already corrupted when Corypheus' posse entered it.
Jaws of Hakkon and the Descent did a good job of giving answers to a lot of the questions it raised, while leaving enough open for us to speculate and for Bioware to build upon in the future.