Also I'm pretty suspicious of Gladwell. He spins a good yarn but his work is less than rigorous.
Historically, the marked exploiting cheap resources was so pervasive that we generally needed the government to prevent it. For example, to prevent employers from hiring cheaper negroes over whites, we passed Jim Crow and Davis Bacon laws.
https://en.wikipedia.org/wiki/Davis%E2%80%93Bacon_Act https://en.wikipedia.org/wiki/Jim_Crow_laws
Of course we assume, in this thought experiment, that all the other things Company Bigot could do to become more competitive (invest in more employee training, better infrastructure, etc) are also available to Company Diversity. This isn't always the case. Maybe Company Diversity's hiring practices have alienated it from potential suppliers or clients who prefer Company Bigot's views.
Specifically I'm thinking of Japan, where (apparently; source: this site) the cultural emphasis on the salaryman paradigm means that hiring from the significant pool of contract labor can hurt your company image as far as working with the established corporations is concerned.