I guess what I meant was that-- logic (to me anyway) would put an existing engine inside the big hole created by the frame, or on the back like a rack mount... but instead of taking an existing engine and putting it somewhere on the bike, they created an engine that follows the existing lines of the frame of the bike-- leaving the "big hole" empty... like the "big hole" is important or something.
I see it as-- they went out of their way to keep it looking as much like a pedal-only bicycle as possible, "hiding" the engine along the frame of the bike