I believe the two foundation stones of the special theory of relativity are the principle of constancy (Michelson-Morley) and the principle of relativity (Galileo).
1. The velocity of light 'in a vacuum' is the same in all frames of reference (for al observers) moving uniformly, relative to each other,
I am not certain that the post mentioned that the car was being driven in a vacuum?
2. All laws of nature are the same in all frames of refernce moving uniformly, relative to each other.
The speed of light only 'appears' to be constant doesn't it? it is the measuring instruments that change from one frame of reference to the other in just such a way that the speed of light appears to be the same.
