BecomeMe

That wasn't the conclusion of the study at all; It only sought to establish if there is indeed a correlation between between diversity and a company's EBIT margin (essentially, a cost management index) in three seminal McKinsey studies. It did not aim to discuss the effectiveness of diversity in corporate hiring in a broader sense:

While our results do speak to the lack of robustness of McKinsey’s (2015, 2018, 2020) studies vis-à-vis large public US firms, they do not speak to the connections between racial/ethnic diversity in employees and/or Boards and either firm financial performance or non-financial firm goals, nor to intra-firm activities. Nor do they speak to any social or moral contributions that racial/ethnic diversity in US executives provide.

We conclude that in light of the prominence of the connections between firm financial performance and the racial/ethnic composition of their employees, not just in the US but around the world, there is great value in future research that would seek to empirically test for the presence, sign, magnitude, and direction of any causal relations that exist. Such longitudinal and causality-oriented study may also help bring into sharper focus the identities and sizes of the costs and benefits, as well as the risks and returns, that are associated with higher or lower racial/ethnic diversity, not only in firms’ executives, but in their Boards of Directors and rank-and-file employees.