It is legal to operate a car with only one working taillight in that state however if the car has another taillight fixture and it is not working then there is a violation.
Interesting.
It is possible, then, that the stop was perfectly legal, but that a defense attorney was able to throw enough talcum powder in to the fan to cloud the issue.
Unless we know what kind of car he was driving we don’t know for SURE, but... interesting.