Next time you get the chance, spend a day sitting next to a controller at a busy approach facility, and then tell me you still believe "humans are not capable of multitasking".
Is a busy air traffic controller multi tasking? Or just performing
one task - maintaining safe air traffic flow? I think you'd find that air traffic controllers, while on duty, are very insulated from distraction, and any multi tasking. Even sitting beside one comes with limitations for the sitter.
On the other hand, a pilot, flying an advanced fast airplane, is tasked doing that. Then that pilot is thinking about maneuvering into formation with another aircraft, another task with constantly changing parameters, and [hopefully] considering his own traffic separation - many distinct tasks - 'cause he has to look all over the ski for them, rather than at one radar display.
I entertain the notion that the Air Cobra pilot was task saturated, in a very complex situation, and could not keep all the balls in the air. I have had a few occasions in an uncontrolled airport environment where non conforming traffic was too much to mentally track, while I was also PIC, training a pilot new to the type, who was flying. I chose to overshoot a visual approach, and reorient myself with the traffic, before attempting another approach.
In my opinion, the set up of this flying display was a major Swiss cheese hole toward task saturation, and needlessly so. As said, great display antique airplanes, but not in a highly complex multi plane formation in turns. Just fly them one after another across in front of the spectators - and, then they can also enjoy the distinct sound from each one too!