"Whether or not you use the reducing back or the full 8x10 back, the intensity of light hitting the centre 4x5 of the film is exactly the same." -- Agreed!
"If you use the 210mm lens designed for 4x5 on the 8x10 camera, for a given subject distance the centre 4"x5" part of the image on the 8x10 film will be substantially the same as if you had used the 210mm lens designed for 8x10." -- I'd go one step further: the object size on the film would be identical for both lenses. In other words, the image on the 4x5 film using the 4x5 lens would be exactly the same as if we had used the 8x10 lens with the 4x5 reducing back. (The key difference: angle of view.)
But the light coming through the 8x10 iris is being spread over four times the area. If the intensity per unit area is the same with both lenses, the surely must mean four times as much light energy came through the 8x10 iris, even though it's no larger. Yes?