This render was not done in GT6, it's even possible that they have to completely re-model it for the game, including animations and working gauges. Stuff like that takes time and it seems like either PD, Mercedes or both wanted to add the car day 1, no matter what.
I'm guessing PD have had considerably more time to work on the exterior, since that was almost certainly the first thing Mercedes finished. Especially if the current interior design render wasn't completed until a few weeks ago. But we're still in the dark on that.
Last I heard, they were actually including two versions rather than making a single customisable car. One a fixed wing variant like we saw in the Mercedes trailer, and the other based on the real concept.
It took me a little while for me to really like the car, but it grew on me and I think it looks fantastic. The rear reminds me of the old streamline trains from the 40's like these:
Maybe it's just the bare metal look that reminds me of the trains, but there's just something about the rear of the car that gets me thinking.
