In the overall population, bedside manner and medical aptitude are likely uncorrelated. But the individuals that fall into the quadrant of bad bedside manner AND low medical aptitude will be filtered out of the profession. That means that in the remaining population, you have an externally-induced negative correlation between bedside manner and medical aptitude.
So if you find a doctor with bad bedside manner, they're likely to have better medical aptitude otherwise they would've been filtered out.