Yes, that was my first reading, too! However, it doesn't SAY that both vessels have to be underway, only that they have to be approaching. Still, my study guide says, specifically, that an anchored vessel may sound the danger signal. I'll try to get some further clarification on that from the instructor (a retired Coastie) and will post his response.
You're right about the wording of the Rules. Sometimes I think they were done either by a committee of non-English speakers or by someone schooled in the art of obfuscation. There are many examples where they are not immediately clear, even with some study. And I've spent a lifetime on the water, and been a licensed master for over 25 years (though not unlimited like you!). I OUGHT to be able to understand this stuff straightaway :-))