(I cannot distinguish product development from engineering as cleanly as you do, but on sales and marketing...)
Can't disagree more. At least with the definitions I have in my head, this sentence is logically inconsistent with such definitions. Engineering is the way you get non-linear leverages. Sales and marketing, almost always give linear multiples at best.
I feel once you have the "golden goose" leverage delivered by engineering in hand, you start calling it sales because that's the final step in conversion to $$$. Also, that's when the linear multiple provided by sales and marketing is the biggest in absolute numbers.
[But yes, if you call everything you see as "marginal contribution" engineering, and everything valuable "product development", then you tautologically "prove" your conclusion.]