We don't actually know if that's what the data shows. For one, diversity can refer to age, race, national origin, etc. They only refer to sex, all those other forms of diversity could actually be a negative for all we know.

Additionally, it's possible teams with 100% female founders do better than ones with a mix of sexes among the founders. That would mean again that diversity is not good, but being female is.

Obviously though, this is an n of 300. I would guess that includes less than 100 companies with female founders, making it not exactly proven.