So they claim. A few stock Skyrim characters mention the Dawnguard and suddenly this 'expansion' of 10 hours 'builds upon the main game.'
You have no idea what their conversation was about do you?
The poster was thinking the DLC would be like the DLC for the GTAIV which was a standalone stories whose content wouldn't carry over into the main game. You'd be a different character with different weapons and abilities made available.
ChapTES was correcting his misinformation. Now I will correct your misinformation as well.
Typically DLC for bethesda games contained new map areas, you could only access them at a fixed point in the main map. Once the DLC's begain you normally couldn't leave the new area until completed.
Dawnguard adds new areas to the existing map, actually expanding its boundries, in addition to vampire lords, normal vampires are getting an overhaul. New options in weapons and powers are being added which can be used anywhere. How does that not build upon the main game?