"For the purposes of these numbers I’m not taking into account ‘differentials’"
Contracts based on a differential against the C price is how most specialty coffee is purchased. Leaving them out makes an already fairly hand-wavy analysis less than useful.
Interesting article, though the final graph and analysis is a bit goofy. He arbitrarily fixed the price of specialty coffee, and then asserted that specialty coffee allows more stability. Well yeah, if you magically remove a major source of variability then of course things are more stable.
In reality the larger margin on specialty coffee should provide the farmer with more breathing room. But I doubt that it's the sure thing he's made it out to be.