On a lens datasheet like this, the listed exit pupil value is typically the exit pupil position, not its diameter.

For compound lenses, positions are referenced to the lens’s optical principal/nodal points rather than a physical glass surface. In practice, image-side distances are measured relative to the rear principal point (or equivalent image-side reference used by the maker).

So -57.9 mm means the exit pupil is located 57.9 mm from the image-side reference point, with the negative sign indicating direction according to the sign convention used on the datasheet. In the explanation given by the community, the negative sign means the exit pupil lies on the opposite side of the reference/image direction (described there as to the right of the apparent image).

In short: it is a location coordinate, not pupil size at a certain f-number or focus setting.

That’s also why it doesn’t directly match your pupil magnification calculation: pupil magnification relates the sizes/positions of entrance and exit pupils, while the datasheet’s -57.9 mm is simply giving the exit pupil’s axial position under the manufacturer’s optical sign convention.