From the last thread:

Quote Originally Posted by SurlySeraph View Post
Well, until that loophole is formally closed, we have evidence that Primordials can in fact become immune to their weaknesses.
Well, the thing to remember about the ebon dragon is that his weakness was his own deliberate fault the whole time, given that he literally invented the concept and embodiment of goodness so that he'd have something to pick on.

And possibly because he's incapable of doing anything without stabbing somebody in the back and he considers himself a valid and comical target.