Er No, the bottom line is you're BOTH breaking the law if you're riding without lights in poor visibility. I agree, you are correct, drivers SHOULD slow down BUT if a cyclist has no lights he SHOULD dismount. Sorry to say,as the OP found out there are a fair number of cyclists out there with a massive chip on their shoulder who don't believe the rules apply to them.
There's no need to shout. I'm aware that the cyclists should have had lights, and I also think it is a no-brainer to carry a small rear light. But the reason I used the phrase "the bottom line" is because if a car driver is taking sufficient care, he will not hit anyone regardless of how unwisely they are riding. Most groups of cyclists will use lights in the fog, and not doing so is mainly a compromise to their own safety - whereas driving too fast for the conditions is endemic and endangers others. I'm merely talking about putting the onus on the more dangerous party.