I know its an old post…
I know this is kind of an old post… but for people searching in the future, I imagine that the people at Epic Games had designed this character originally with custom shapes for their bones. Animating with custom shapes makes the whole process quite a bit easier. If you go through and make custom shapes for each bone it may make the armature bone structure easier to understand such as a torus shape for most arm, digit, and leg bones. This helps tremendously with the most recent mannequin too.