Basic colour meanings of red and green transcend many genres of human existence, from business (using a red pen, or being "in the red") to entire gamuts of electronics and engineering, red warning stickers, etc.

I agree that the octagon is arbitrary. But that's beside the point, because it's self-evidently arbitrary. Nobody assumes there must be a deeper meaning behind a basic shape. The iconography of a floppy disk is not a basic shape.