Questioning witnesses is literally the DA's job. That doesn't magically stop being the case just because the witness is an expert. The DA is indeed qualified for this. You could even say it's one of the main reasons the DA exists.
The author of this post admitted that it took him countless hours to discover that medicine basically has this issue wrong and only after being particularly motivated to do so. If the DA has so much medical expertise that they know better than the doctors, then what exactly is he doing being a DA?
The idea that the main purpose of the DA is to know better than doctors is ludicrous.