U r right about people doing it on 4WDs on narrow roads but they dont have to worry about balancing, to ride a bike on such a single hiking trail is totally a different thing. It is lot more difficult then it looks.
Easy said then done buddy. The link for this video was posted on a dirt and off road motorcycle forum, even the most seasoned and hardcore riders didnt think that they could do it. This requires extraordinary guts and skills. One rider said and I quote "Its easy to ride a narrow lane in a park and I would do it million times but replace the grass on either side with just six feet of drop and it is a whole different story."