Same experience here. I've already owned the base game, but none of the DLC. It let me accept Spacer's Choice, which is literally just another store entry for the base game, but you can buy the upgrade for 10$ instead of 17$.
Update: I've done some research. Apparently this version of the game is like a third-party remaster done by a different company, hence why it's a different entry in the store
EDIT - Close to the add button Is written [Base game] and even after adding it to my library i can still see the 9$ dlc option in the menu. Either way is not too clear so I'll let the (base game) in the title.