I have spoken with MS extensively over the last few days, by chat and email. They are adamant that the DLC is standalone and not included in the Game Preview pre-orders or other digital pre-orders for Xbox.
I have emailed Larian (who has confirmed it should be in XGP and digital pre-orders) and they stated they are in contact with MS but no ETA on a solution.
MS was really frustrating; originally they were trying to tell me two versions of the game were on Xbox and my purchased version was incomplete, waiting for Larian to update it. After pushing back (and verifying there was only ONE version) I was told to contact Larian ,which I had at that point, and after telling them as such, they then told me to contact Bandai Namco.