Maybe it is and maybe it isn't, but I don't argue on these grounds because it's not the reason I think what I think and it would not change my opinion if the research broke the other way.

Then we don't need to social engineer it. Or worse, mandate it. Both of those strategies ALWAYS have negative side effects.

Do businesses always behave perfectly rationally?

Farrrrrrr from it. Humans don't behave perfectly rationally. Humans often behave badly, in fact. I think you'd agree businesses are just humans (as are governments). That means they get all the beautiful and ugly traits we bring to them. That's why a strong legal framework to prevent business/other humans from using coercion against you and I is a great idea! That is what government is for!

So then it doesn't follow that there is no need for intervention simply because it is in businesses' rational interest to promote diversity.