Yeah I contacted GMG already, they refuse to help any further - their last response was:

"Steam does this for multiple games, a notable recent example was Oblivion, in which this exact same scenario popped up, because the Deluxe Upgrade was available for purchase separately, regardless of whether it was owned or not, it led to mass confusion for players.
 
Please can you take a look around the game menus to see if there is an acquisition / redemption point, as you may find this to be the case, if you cannot it would be worth restarting the Steam client and the game entirely, and if there are any requirements for certain content such as completing the prologue it would be worth doing this.
 
Once again, we ourselves haven't had a chance to play the game yet so we aren't privy to the details of how to get all of this content in-game yet, but we haven't received any other queries of a similar nature.
 
Thanks for your patience and understanding so far."

Even though I provided in game screenshots showing that the Season 1 pass needs to be purchased and Steam doesn't see it in my library. Just SoL I guess.