Towards the end of the movie the Union doctor and the hot chick are left behind with the wounded as the Confederate calvary appears. The commander of the calvary says "Doctor do you need any assistance?". How could he have known that he was a doctor?
US Army shoulder strap rank insignia were color- coordinated, with different colors for different branches of the Army. For example, yellow for cavalry, red for artillery, blue for infantry. Medical Corps was black, with MC embroidered between the rank insignia. Any experienced Confederate officer would know how to read rank and branch insignia for the Union Army.
Just watched the movie. When Kendal is speaking to the Confederate Officer he is in his service blouse with the rank tabs attached, and not wearing an apron. Earlier, when he is operating on Marlowe's leg you can see the rank tabs in a long close up. They are colored either dark green or black. There is no MC embroidered on them.
According to Wikipedia, in the US Civil War the background color for shoulder straps was emerald green for the Medical Department, and Kendal's background color seemed clearly green in at least one shot as I watched the movie a few hours ago.