The owner of the GT500 posted on the forums saying his car is stock and that the F10 M5 belongs to his friend who is still figuring out the various shift modes. It's honestly pretty simple, put it in the highest shift mode and then push it to the right for automatic mode which will execute a perfect shift every time. Additionally, the F10 M5 doesn't need to be wound out super high with its torque happy turbo motor so it's hard to explain why it lost the way it did.
In the first race the GT500 seems to get a jump and the M5 gets demolished. The second race is much closer but the M5 loses again. What happened in the video is what happened but hard to figure out why the M5 with its 527 wheel horsepower stock got beat the way it did by what is said to be a stock 2011 GT500. Video is below.