True, however his story really didn’t have much to do with the core “love triangle” narrative, and thus felt “tacked on” to an already too-long film.
That’s probably true for this particular movie. The overwhelming majority of soldiers at Dunkirk were white and telling the story of one of the non-white soldiers would have been atypical of the overall event.
But the argument looks a lot weaker when you see it used against a movie like The Magnificent Seven remake. People complained about the multiracial cast in that movie using the historical realism argument. But actual historical realism would tell you that a lot of people in the old west were non-white. The cast of the 2016 version was more historically realistic than the cast of the 1960 original (or most classic westerns).
And let’s not get started on the people who invoke historical realism in defense of all-white casting in science fiction or fantasy films.
A lot of people who try to argue for historical realism aren’t talking about actual history. They’re really saying they want the casts of modern movies and TV shows to look like the casts of movies and TV shows that were made sixty years ago.
Cuba depicted one of the bravest sailors of Pearl Harbor so yes, it did in fact improve on the film.
The French Army at Dunkirk included a substantial number of soldiers from Tunisia, Algeria, and Morocco. Most of these soldiers were not allowed to join their contemporaries on the evacuating boats and were left on the beach to be captured. 25% of crewmen of British merchant vessels were lascars from Asia and East Africa. They of course participated in the evacuation. Three Royal Indian Army Services Corp transport companies were on the beach. Not one or two Indian soldiers, several hundred. So, yes, there were far more than “six black guys” at Dunkirk and no, the witewashing of Dunkirk and WWII movies in general is not politically correct “nonsense”. WWII was in fact a global war, and a multicultural one at that.
The “wingman” combat doctrine is a real thing, originally developed back in WW1. (See also Dogfight - Wikipedia)
“Target fixation” is also a thing, and being displayed by the German in your description.
My assumption is that the trailing Spitfire (aircraft C) did not want his rounds hitting the leading Spit (aircraft A) while he shoots at the German (aircraft B), so he has the ‘A’ spitfire bank to the side in order to clear the field of fire (of ‘C’).