It is very unclear that to be the case. It is mostly attributed to a study of McKinsey, and as we all know, everything McKinsey does is flawed. There have been some studies recently showing the exact opposite, albeit from quickly skimming them, they look kinda bullshit.All the stats show that more diverse executive teams perform better. Now obviously there's a limit to that - they still have to have relevant skills, interests and experience - but pushing for a deliberately more diverse representation in leadership is shown to benefit organisations on average. By all means it can be taken too far but the stats show that it rarely is, and much more frequently the opposite still.
This is a very nice essay on it which tries to be quite balanced: https://medium.com/@alex.edmans/is-there-really-a-business-case-for-diversity-c58ef67ebffa
Personally, I think the diversity of thought is far more important than other forms of diversity for a company to be successful. You can have different genders, LGBT, people from different countries and races, but if you create an echo chamber of thought, you are going to massively suffer. Which has been what lots of companies have been doing for the last few years and then crying 'bigots' when their products got ridiculed (Google Gemini most recently).