That is true, however I think they are attesting that he could not keep clear b/c he didn't see them b/c they didn't have their lights on.

The nearest I can come up with an analogy is this:
You're driving down the road at night with your lights off and another car runs a stop sign and you collide. Who is at fault? He could not see you, but he also ran a stop sign.

I've spent a LOT of time boating at night on boat sailboats and large and small power boats... I'll admit... in several cases we've been going a weeeee too fast (70+) and you can't see a lot. Day markers and running lights, but that's about it. You have to be very careful with any lights b/c white lights can easily blend in with lights on shore. So even if their lights were on... it might be difficult to see.