Can getting a sunburn really get you an article 15?

I nearly did in 1974 but got the burn on my off duty time and the the thing that saved me was I went to sick call but did not miss any work even with 2nd deg burns, fortunately I worked in supply and could sit at my desk and gl;ow.

Yes, if it prevents you from doing your duties.

Yrs ago, 1960 sunburn was grounds for Article 15. I knew one Marine, fell asleep on Onslow beach, Camp Lejuene NC . Needless to say he blistered, and lost more than a week of duty. Loss of 1 rank and $50.

If an NCO/Officer wants to be a dick they can try to give you an Article 15 for a sunburn if you get one severe enough to prevent you from performing you duty through your negligence. I knew someone who did. He fell asleep on a beach and got so sunburned he was in the hospital.

As a commander, unfortunately I had to administer some Article 15 and I am familiar with the different categories, but "immediate Article 15" does not ring a bell. First it implies lack of due process, second, the NCO does not have the authority, and most important, unless he locks you up in a room without windows, he has no way to ensure you don't get a sunburn. I doubt your commander would agree if it were brought before him.

Shortly after being assigned to Albrook AFB, CZ back in 72 I went out for a day on a sport fishing boat. Not being used to the tropical sun (I was from NH) I wore only a sleeveless shirt, cut off fatigues and a baseball cap. I got so sun burned that the next day I was covered in sun blisters and spent 2 days in the hospital. I don't remember getting an Article 15 but I do remember that the 2 days I spent in the hospital was considered "lost time". I didn't get paid for them and, in theory they were added to my enlistment but I reenlisted so it didn't make a difference. I learned to dress a little better the other times I went out on the boats.

misconduct not in the line of duty (not protecting skin while on liberty), assuming you can't fully perform your duties

Is this a commanders policy? NCOs dont administer UCMJ

You can be written up for yes. Destruction of government property. There's a couple more I remember just not the correct wording.

I once heard of that happening years back, through a civilian speaking for a friend.
I'd be more concerned that your bosses are managing people through the use of threats and potentially draconian punishments. That isn't discipline, that's an extremely petty reign of terror.