“he says that it medicinal matters we hold like the scientists over Chazal”
Actually that halachic principle did not originate with Rambam; it appears in the writings of Rav Sherira Gaon, who lived centuries earlier.
If in medical matters we hold by contemporary medical experts over Chazal, kal v’chomer we hold by contemporary medical experts over contemporary rabbis based on yeridot hadorot. However, this should not be interpreted as saying that rabbis have no role to play. Here is an example: I know a frum couple whose first child was born by caesarian section and they were not sure whether they could have a second child via natural birth. (The husband is a physician but not an expert in obstetrics.) The medical evidence was not compelling one way or another (and this depends a lot on the individual woman and the type of c-section performed) and it was not clear what the answer should be. Their rabbi was able to discuss intelligently with them how the Torah views risks and benefits, and empowered them to make a decision based on Torah views in the presence of the evidence. At no time did their rabbi pretend to be a medical expert, and he did not make the decision for them.