Frankly, I'm much more bothered by the idea that McCoy would know the anatomy of any species other than human well enough to give medical care. Mastering just one species is a lifelong task for us today...
Then again, which alien species, apart from Vulcans, did McCoy successfully treat, for anything besides the most banal physical trauma? He patched up Ambassador Petri's knife wound, but other aliens like Lazarus or Lokai essentially healed themselves, with the good doctor merely venting his amazement and frustration.
I don't see how McCoy would be capable of treating Gorkon unless he was part of the special Starfleet Medical unit that experiments on captured Klingons.