I think Nikon is right. The D800E will have a larger diffraction limited aperture when it is calculated based on the pixel level resolution. If based on the desired print resolution then there will probably not be as noticeable a difference.
Diffraction causes light passing through a hole to spread out into a cone. This falls onto your sensor pixels. When the cone is larger than a single pixel then the diffraction will cause light to fall on more than one pixel. So for a sensor with no anti-aliasing filter the diffraction limit is specified by the amount of diffraction that causes a point source of light to fall on more than a single pixel.
When you put an anti-aliasing filter on a sensor this spreads out light falling on one pixel to the surrounding pixels anyway. So you already have light falling on multiple pixels. So for a sensor with an anti-aliasing filter the diffraction limit is specified by the amount of diffraction that causes a point source of light to noticeably fall on more than the original number of pixels. How to define 'noticeably' is varied.
This equates to having a bigger pixel size to define the diffraction limit. So with the filter on you can state that the diffraction limited aperture is smaller (higher f-number).
However the exact specifications of the anti-aliasing filter are needed to calculate the effect. Also you need to specify how you will define a noticeable difference. Due to the vagueness of this I believe that diffraction limit calculators do not factor in the anti-alias filter and simply use the raw pixel size.
In reality the smoothing of the filter and the gradual expansion of the airy disk due to diffraction will cause a light smudging at the pixel level. This will increase as you go beyond the diffraction limit. However I believe that your photo will benefit more from the increased depth of field than it will suffer from the pixel level lack of sharpness.
When you shoot on film there is no pixel size. In this case the diffraction limit calculation is based on the acceptable circle of confusion, i.e. how much can a point of light be spread out before the human eye can notice it.
You can try a diffraction limit calculator here:
https://www.cambridgeincolour.com/tu...hotography.htm
This calculator will show you when your image will be diffraction limited for a real print at a real viewing distance. This is a far more useful limit than the theoretical limit based on pixel size. It will also allow you to work out the limit for your medium format film.
If you have a view camera you have the advantage of moving the plane of focus using tilt. This can overcome softness problems with high f-numbers by allowing you to use the lens in the sweet spot. I don't want to encourage you to spend too much money but you could look at the Nikon tilt-shift lenses in order to satisfy your landscape shooting needs.
I am sure you will enjoy either D800. Digital is a lot more convenient than shooting film. But if you are not sure you could wait and rent one (but unfortunately lose your place in the buying queue).
Alex