I understand skepticism, but the fact that you don't think U.S. military aviators, the most well trained pilots in the world know what a weather or research balloon would look like is disingenuous.
Cruising speed of an American Fighter Jet is around 609 mph.
Objects at that speed would be hard to identify. Radar helps, but all that tells you is that there is a thing there.
Think about you in your car. (assuming you drive a car in the USA)
The highest speed you have ever legally been at is 70 mph. Multiple that by 10 and think of all things on the side of a highway you couldn't identify but thought you knew what it was.
I've illegally driven at 120 mph once and while dodging cars on the highway couldn't tell you what I saw on the sides.
The human eye and human brain can only process so much, especially at speeds like that. Just slightly slower than the speed of sound. Our brains fill in the dots.
The image is not a cube, as the pilot described it.
It seems a case of an unusual object, a balloon of some variety probably entered their line of sight, they got a picture and it wasn't a cube.
Although maybe it was just Thor's birthday.