Kind of ironic that the patient being argued about is, in fact, very dead and the doctor being questioned is a doctor that cannot kill his patients. Dr. Horn is not on a pedestal and no one, incuding him, is saying he's infallable.
But, he is the M.E. who conducted the autopsy, his opinion holds more weight than any other person, especially speculators who are not qualified to render a medical opinion in this or any case. Whether that is acceptable or unacceptable, Horn's opinion is the legally valid one, his signature is on the official report, he's considered the expert witness (at least so far), and his opinion on the condition of the body and the analysis conducted on that body is one a jury will be expected to consider with greater weight than, say, Flores or anyone else.
Of course anyone can believe anything, including a theory that aliens flew down in their spaceship and committed part of the murder, but they're not sitting on the witness stand, under oath. Their opinion means nothing, legally, and has no bearing on the case.
In other words, it's neither here nor there. There's only one expert legal medical opinion in this case (so far) and he rendered his expert opinion and that's what the court is going with.