> But taken on its own, his research certainly is evidence that diversity is bad.
To be more precise, it says "diversity has drawbacks."
An important distinction, because when side A says "diversity is good" and side B says "diversity has drawbacks," then side A can come back and say, "well here are the benefits." Then you can compare the benefits and drawbacks and see if that helps you arrive at a better decision.
If side A says "diversity is good" and side B says "diversity is bad," then you have an intractable conflict.
Or worse, what often happens is that side A says "diversity is good," side B says "diversity has drawbacks" then side A says "side B says diversity is bad, they are evil you should not ever listen to side B under any circumstances."