First: Sony doesn't necessarily disagree with Nikon's claim. It's just that Sony designed their 'E' mount with a throat diameter of 46.1 millimeters at a time when it appeared it would be an APS-C only mount for the NEX series of compact mirrorless ILCs.

Sony later made the decision to move into full frame territory using the all-electronic 'E' mount, rather than either using the existing mechanical 'A' mount¹ that dated all the way back to Minolta film cameras or creating yet another new mount for their FF mirrorless cameras. The 46.1 mm throat diameter of the 'E' mount is just large enough to accommodate the 43.27 mm diagonal of a 36x24 mm FF sensor.

Second: The all-electronic Canon EOS 'EF' mount, introduced in 1987, has a throat diameter of 54 mm. The existence of a 50mm f/1.2 lens in the Canon EF mount with its 54mm throat diameter does nothing to disprove Nikon's claim that their new 55 mm wide 'Z' mount allows better lens design than their previous 'F' mount, which had a narrow throat diameter of only 44 mm.

Basically, since 1987 when Canon introduced the 'EF' mount with a 44 mm registration distance and a 54mm throat diameter, Nikon has been technically limited from matching some of Canon's lens designs due to their own 46.5 mm registration distance and narrower 44 mm throat diameter.² Similarly, with the introduction of the Sony 'E' mount in 2010 and the application of it to FF cameras with the introduction of the Sony α7 series of cameras in late 2013, Nikon 'F' cameras (and Canon EOS cameras as well) were at a disadvantage with regard to designing very wide angle, very wide aperture lenses that can take advantages of the shorter registration distance to both simplify design, reduce size/weight, and match or improve lens performance in a smaller package.

Nikon is touting the newer throat diameter and much shorter registration distance of the 'Z' mount because it is one millimeter wider than the Canon EF mount has been since introduced in 1987 and two millimeters shorter than the Sony 'E' mount has been since introduced in 2010. It's also 11 mm wider and 30.5 mm shorter than their own 'F' mount.

For shorter focal length lenses with wider apertures, a larger throat diameter allows larger exit pupils. A shorter registration distance allows shorter focal lengths without needing to resort to a complex retrofocus design to make lenses with focal lengths shorter than the registration distance. Both of these factors combined mean that larger rear lens elements can be positioned closer to the imaging sensor. This allows for lens designs not possible using narrower throat diameters placed at greater distances from the camera's image plane.

Control lens. 85mm f/1.0

With an 85mm focal length lens, the difference between 16mm and 46.5mm registration distance is not really a factor at all because 85mm is considerably longer than even the 46.5mm registration distance of the Nikon 'F' mount. When one looks, for example, at 85mm lenses for the Sony E-mount and compares them to 85mm lenses with the same maximum aperture for the Canon EF mount or Nikon F mount, it's pretty easy to see that the lenses are about 30 millimeters longer for the 'E' mount to make up for the roughly 30 millimeters shorter mounting flange. The rear elements of the 85mm lenses for the Sony 'E' mount are about 30mm more recessed into the lens.

The wider throat diameter is a factor, because it allows light striking the edge of the imaging sensor to strike it at a more perpendicular angle than a narrower throat diameter would. This increases the amount of light falling on each photosite in the same way that each square meter of ground on the surface of the Earth receives more light/energy from the sun when it is high overhead than when it is low on the horizon. In fact, it would be even more analogous to how much sunlight, based on the angle of the sun, would strike a one meter square on the Earth's surface with a solid 1-2 meter high fence around it, since the pixel wells on typical ILC sensors have depth that usually exceeds their own width.

¹ The Minolta/Sony 'A' mount had a 44.5 mm registration distance and a 49.7 mm throat diameter.

² The previous Canon 'FL' and 'FD' mounts had 48 mm throat diameters that were 4 mm wider and sat 4 mm closer to the film than the Nikon 'F' mount, giving them a slight advantage when designing very large aperture lenses in medium focal length ranges. That's one reason Nikon's 58mm prime could be designed differently than their 50mm primes. The 3.5mm difference between 46.5mm and 50mm is not quite enough room to fit all of the lens elements needed for a higher quality yet simply designed 50mm lens.