The resectioning pixel grid is relative to the pixels in the output image being evaluated. It is not inherently dependent on the image sensor's pixels/photodiodes per se.
I.e. you can have pixels with .5a sensitive area (e.g. masked phase detection photosites) and the other portion of that photosite is calculated/mathmatically replaced in the output (or the ...
I'm not familiar with camera calibration, but in any case focal length, the real focal length, does not depend on shape or size of the sensor.
Field of view does depend on shape and size of the sensor. And often we use 35mm-equivalent focal length as a measure to compare the field of view of different setups. But that's not the real focal length.