I haven't played the IJ game or seen any details about its DLC, but DLC can sometimes be accessed before the main game is completed.
It could potentially be like the Dark Souls 1 DLC, where you can enter the area at any time after a certain point in your playthrough. You can enter Oolacile after you defeat the Hyrda in the Basin and the Crystal Golem in the Duke's Archives. This can be completed at various points in the game before you finish the game.