Calm down mate, it is included. I had a look, and initially thought the same thing, but I remembered reading somewhere that the Porsche packs were included in the season pass, so I clicked on the DLC in the PS store and read in the description that Season Pass holders were not to purchase it from the store, as it would be automatically available to them in game after the patch. The exact quote from the DLC description is "Do not purchase if you have already purchased the Assetto Corsa Season Pass - this DLC will be available at no additonal cost for Season Pass owners."
Edit: They've set it up differently to pretty much every other game. In Driveclub, for example, if you had the season pass, every DLC pack would simply say "free" for the price when they became available. In AC, it seems the update brings the DLC contents to the game, but they're locked out if you haven't puchased the pack. If you have purchased the season pass, the new content will always be available without having to do anything, after each DLC patch.