The wonderful thing is all the achievements tied to the DLC that's already not purchasable. Tough luck if you want to 100% it now and haven't bought the dlc.
(Which I don't have. Sad face.)
This is pretty common racing game happenings though. Especially AAA titles.
It's like publishers want people to pirate games. We know that's surely not true, but their practices seem to make it the only option to appreciate the entirety of the game, and eventually the game itself.
My mistake. I tried to purchase Ultimate edition in the past and it wouldn't allow me to since I already owned the game. Guess they were trying to force buying the DLC piecemeal.
It is purchasable now. Just get the "You won't get an extra copy of the game." error.
To add to what you are saying, I wish they would make that message clearer. The first time I saw it, I thought they were still going to charge me for the extra copy and just not give it to me. Still not 100% sure about it.