Undead nightmare is a separate game completely so it isn't considered DLC, it's just a Download Title.
It's really a simple thing when it comes to that... it's not DLC. If the title doesn't require the main game then it isn't DLC. It shouldn't even be a topic of discussion really, it should be treated no differently than any other digital only title.
I think the discussion in general is a dangerous discussion because allowing stuff like skins and such as separate listings will have people listing stuff like the 3000 hats in TF2 in the database with no one to monitor it because only one user cares about it and no admins know better to audit the listings.
Edit: Having read through the posts in reference, I see the conversation is about more than Undead Nightmare. DLC is a complex conversation. I think, personally, it should be notes on the item in the collection until such a point that a more robust system could be put into place for the database. A separate entry system for DLC or something to link back to parent items with cross references, or integrating it into the existing system requiring a parent item to submit dLC entries. Not a simple change, but likely the optimal solution.