optics - How does a short flange back distance improve image quality?

In the description for the new lens mount for their X-Pro series, Fujifilm says:

1: Specifically designed to maximize the mirrorless design of the body, the X-Mount has a short flange back distance of just 17.7mm. This means the rear lens elements are as close as possible to the sensor. The wide opening allows the lens to be mounted deeper within the body - up to 7.5mm (approx) from the mount surface - reducing the back focus distance of each lens to the minimum possible, thus achieving high resolution all the way to the edge of the image.

How does this shorter-than-usual distance help "achieve high resolution"? Does it come at a cost? I remember both Olympus and Leica making a big deal about the issues with non-parallel light rays at the edges of digital sensors, and how that caused a sort of vignetting not found with film (where the emulsion is basically non-directional). It seems to me that a shorter flange back distance would force light rays outside of the center of the frame to be even more oblique. Am I misunderstanding how this works?

The Sony E-mount is almost as close. Is this shorter distance all advantage, or does it bring problems as well? (Clearly, it means all-new lenses, but it should also make adapters easy.)

Answer

Having the lens sit much closer to the sensor removes the need to have a retrofocal (reverse telephoto) group which results in less extreme image correction, fewer lens elements and often a sharper image.

However the increased angle of incidence of light rays at the edge of the image often results in increased light falloff (vignetting). For this reason a weak retrofocal group is included even when a non-retrofocal design is permitted.