I'm only concerned about the 21.6mm diameter of that image circle that would correspond to the maximum diagonal of any rectangle (or square) that would fit inside that image circle.
Yes on your part. As soon as you restrict one dimension to 15.3, then I can draw a Shiitake Mushroom ton of rectangles or squares that fit in the image circle that do not give a 21.6mm maximum diagonal of "any rectangle that would fit inside that image circle". Perhaps you are talking about only landscape orientation which is different that the original comment about being able to take portrait photos without turning the camera. Sounds like your sensor requires turning the camera for portraits so no real gain there,Anyways, just a small misunderstanding. No biggie.