The release of the pack will be tomorrow, probably around noon CEST.
If you follow some links from SteamDB you can indeed already get to a page where you can buy the pack. We're aware of that and it's a limitation of Steam. The actual cars will be uploaded tomorrow, so you can look at this link as a way to pre-order.
And yes, this evening was one of those days. We practiced this event several times, and still Murphy was particularly active during our live broadcast. Let's just say it was a bit hilarious seeing Rene talk, but not hearing him, while we were trying to tell him his microphone was off. And on such evenings, you then also encounter a problem in Windows as well as this weird bug that had to show up exactly when we wanted to start a live race. Anyway, we learned, we hope you still enjoyed the show. Have a great weekend!