A quick read of the link, I think this undoes any other argument he puts on the page.
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Varying CCD exposure-time or f-ratio varies the number of photons per pixel.
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That, very simply put is exactly what is being talked about. Vary the F number, vary the number of photons on a given pixel in a given time, lower F number = more photons on a given pixel in a given unit of time = faster.
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Focal length (and thus f-ratio) has absolutely no effect on the number of photons collected and delivered.
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This statement is correct but omits half of the equation, it omits how the collected and delivered photons are distributed. Lets say that the entire target fits on the sensor at both focal lengths of a scope, with and without a reducer. The "Faster" ratio with the reducer fitted will take the same number of collected photons from the target and deliver them over a smaller number of pixels (Shorter focal length, smaller image formed) The image will be smaller and brighter as the same number of photons have been distributed over a smaller number of pixels. That is it in a nutshell.