I might put a note on the Wiki page, but I think this is actually a bug. David himself stated the following in the above mentioned thread:
DavidBVal":39vcg85j said:
OK, I found the reason. Please complete the Mysterious Letter quest and you'll be able to dismiss her.
I am not sure why exactly I put that limitation a year ago, but at the time I am reticent to remove it... I probably did it for a reason. While I analize it more calmly, the easy solution is simply to complete the letter quest.
I think this is a classical copy&paste error. Adaon's dialogue has exactly the same limitation in his dismissal line, but for him it makes sense, because you don't want to dismiss him while still in the Unknown Dungeon. But there's no obvious reason for Hirge.
So David, if you read this, I think it's safe to delete this restriction.