It does not study the mortality rate of babies whose mothers had Black obstetricians during their pregnancies.
The researchers did not attempt to identify a cause for this disparity in this study, and noted only that this disparity does not appear to exist for other doctor-patient racial pairings where the mother was not Black.
The point of this study was simply to identify that a disparity exists, so that follow-up research can identify why and hopefully address the disparity in outcomes.