Other cars headlights do create a light source. Even their brake lights do.
You can see their lights shining on the rear of your car in chase cam, and see their lights on your rear view mirror in cockpit view.
Look at this video, at around 14 seconds in you can even see the car infronts brake lights reflecting off your dash. If you watch the video you can then see other cars headlights providing an actual light source.
And at 44 seconds you see the car just to the sides headlights lighting up the interior and wing mirror.
Lastly at around 53 seconds you see the car behinds headlights lighting up the rear of the car.
One thing that can't be argued is the amount of detail they've put into this game, unfortunately I feel that may have been their downfall as it seems they prioritised little details.
My only worry is that they won't support the game the way they have said, but provided they stay true to their word on the weather patch etc I'll be happy.