While I agree with your ultimate conclusion, it's not that implausible that synchronously controlled self-stimulation (which IME most masturbation is, though I suppose it depends on what you're into) is less stimulating than asychronously controlled self-stimulation (e.g., programming a pattern of changing frequencies on a vibrator, or downloading a bunch of porn and queuing a slideshow on my desktop, or visiting a series of previously selected websites with changing content), for many of the same reasons that I can't tickle myself effectively with my fingers but can easily be tickled by inanimate objects.

If that turns out to be true, I would expect a not-very-rigorous analysis to conclude "masturbation is less stimulating than porn", since asynchronously controlled masturbation is relatively rare, as is synchronously controlled porn.