Mr. Koehrer is correct. Lens extension tubes increase the lens to film distance, and so light intensity per square unit of area decreases. However, diopter lenses shorten the lens to subject distance, and do not change lens extension from the film plane, so light intensity per square unit area on the film isn't changed.

If you want to stick with the enlarger analogy, projecting the same size negative with a 50mm or a 100mm enlarging lens, both to the same size final print, will still give you the same light intensity on the paper with the same f-stop.