Of course it makes sense. He swerved off the road and hit a stop sign.
Cops aren't lawyers, but they do know how this stuff works in court. After an accident happens, they can only give their opinions of fault based on what will stand up in court. Since the other driver didn't stick around and confirm that he / she ran your boyfriend off the road, and since there's no damage on his car from the side-swipe collision that he avoided, the cop had no choice but to treat it as a single vehicle accident. That's the only thing that can be proven in court.
If your boyfriend had captured the other vehicle (including the plate) on a dash cam, your boyfriend would have been off the hook. If your boyfriend had simply held his lane and let the other idiot swerve into him, he would have been totally off the hook. But since he avoided the other vehicle and crashed into the stop sign, he's 100% responsible for the whole thing.
Your boyfriend can defend himself in court and pay an expensive lawyer to help him, but the lawyer will tell him the same thing I just said.
Everyone who crashes into a solid object claims they were run off the road by another vehicle. Everyone.