The issue a non-diverse team will hit is getting blind-sided by lack of perspective on emerging phenomena. Given that aspect, I don't find it surprising that the benefits from a diverse team could exceed the benefits between the difference in average and good leadership.
Both an average and a good leader will drown the same army if they lead them into a swamp because they've never seen a swamp before.
A blue collar factory worker, a harvard grad management consultant, and a business researcher who are all white males try to reduce car part inventory through logistical innovation.
A black woman, a white man, and Asian woman who all graduated from Harvard and worked for McKinsey try to reduce car part inventory through logistical innovation.
I wouldn't be surprised if the more diverse group came up with a better solution.
To take your contrived example regional diversity will have a much larger impact on making sure someone has encountered a swamp than racial diversity.
I think it's easy to overestimate the effect of racial diversity and underestimate the effect of other types of diversities on problem solving style because one is so easy to see.
No disagreement here, but if we're talking non-contrived examples, actual facial recognition cameras for video conferencing have gotten all the way to sold on store shelves with an inability to recognize black faces.
I think it's fair to assume that had there been at least one black individual in the development pipeline, the company that made that camera would have avoided an embarrassing mistake.
Both an average and a good leader will drown the same army if they lead them into a swamp because they've never seen a swamp before.