Your analysis is the problem, because no hobbits are needed here.
The point of origin for this light source was determined to be ~24-36 cm to the right side of the camera.
So why would you measure that as 'thumb-tip' to 'elbow-tip' ?
No. They are measuring the distance between two specific points - the camera's centre point,and the light source's centre point.
The elbow-tip is not the center point - it's the outermost fringe.