I think that's just a lie motorbikers tell themselves. Exhausts are pointed backwards. Cars are built so well today that they barely let in any noise. I was surprised so many times while driving a car, because a motorbike was suddenly overtaking me out of nowhere. I can't even hear a motorbike passing me when I am on a motorbike myself, up to the point when the other rider is right next to me.

They maybe help pedestrians and cyclists, but cars, trucks, other bikes etc. don't really hear motorbikes.