"The sum of its parts" is a very opaque phrase here. But even without defining it, anyone with work experience has some understanding of the effect of workplace dynamics, environmental factors, office morale, and other intangible influences on one's productivity and hence, the business's success. In other words, there are emergent properties in the calculus of success, and pursuing diversity is one way of trying to control that.
"Diverse" people do not have to be intrinsically better employees, for them to positively affect the business's success.
Sorry, you put a lot of buzzwords there, but buzzwords do not constitute any proof. It's certainly true in low-skill businesses like , i dunno, agriculture that production scales linearly. If nonlinear interactions occur it would be interesting to name them, instead of attributing it to some kind of magic ether. I dont like this completely vague explanation because it's exactly the rhetoric that people mock when they want to attack diversity
Well, to make it concrete then: I've seen teams get happier and more productive when some asshole is removed from the team; I've likewise seen teams get less productive because someone is added who drains morale, interest and productivity because they're continually negative and waste everyone's time with distracting concerns and a lot of "yes, but what about this?" type navel-gazing. I've found my own productivity fluctuating with different coworkers based on how well I communicate with them (where someone I struggle to interact with well gets along well with others).
So no, you can't view overall productivity as the sum of individual productivity because there are factors like this that are a function of group composition. And there's a lot of factors that are not functions of individual productivity.
Then let’s hear some hard logic. You’ve presented literally nothing.
If the best response you have is handwaving team dynamics as a buzzword, and pretending its not a real and tangible thing, go to work and start abusing all of your coworkers and see if their performance remains consistent.
"Diverse" people do not have to be intrinsically better employees, for them to positively affect the business's success.