The study Rosenberg was commenting on is here, and here’s the important part of the abstract (note especially the caveats that campuses are self-segregated and that other studies have come to different conclusions):

We find that, while much sorting exists at all stages of college, black and white students are, in reality, very compatible as friends; randomly assigned roommates of different races are as likely to become friends as randomly assigned roommates of the same race. Further, we find that, in the long-run, white students who are randomly assigned black roommates have a significantly larger proportion of black friends than white students who are randomly assigned white roommates, even when the randomly assigned roommates are not included in the calculation of the proportions. This last result contradicts previous findings in the literature.

It’s possible that in higher education, diversity — especially diversity that requires interaction, not just being on the same campus — has some impact (good or bad) on students’ racial attitudes and interactions. But right now it’s far from clear, so we should hold off on basing policies on the assumption that diversity causes racial harmony.