This is because sometimes a skeleton has two “root” bones (though only one ACTUAL root bone).
Naming conventions (especially for human skeletons rigged with IK) sometimes names the hip bone “root” and the actual root bone “master”; for example, MakeHuman rigs are this way.
I don’t know about the documentation but I absolutely assure you that the UE4 Blue Man has a “root bone” in the sense implied by Root Motion. In fact if you look at the picture on that page you’ll see the point at 0,0,0 that all bone positions (white lines) are drawn from; that is the Root that Root Motion is observing.