The connector side is almost entirely covered with copper, which is the ground plane for all bus signals.

This is essential, because it ensures every signal has identical controlled impedance.

The STEbus specification states:

7.6.2 Characteristic Impedance

The IEEE Std 1000 Bus is designed to take into account the driving requirements of high-performance transmission-line backplanes. The transmission-line system, together with the specified maximum signal length, allow an accurate determination of the time required for a signal to be correctly received.

Backplanes should be designed using only microstrips for the signal lines, and should provide an unloaded characteristic impedance of 60 Ω ±10% including the effects of plated through holes and connectors.

Backplane signal tracks should have a constant width throughout the length of the backplane so as to keep the same characteristic impedance throughout its length.

Groundplanes are required so as to form a well-defined transmission-line environment. All groundplanes shall be continuous, allowing breaks in the groundplane only around the holes where connector pins must pass through. Under no circumstances should slot lines be allowed to exist in the groundplane, whether in the horizontal, vertical, or any other direction relative to the signal lines.

Microstrip is explained here: https://en.wikipedia.org/wiki/Microstrip

The formula for calculating microstrip impedance is given in

https://www.analog.com/media/en/training-seminars/tutorials/MT-094.pdf

and there are online calculators, e.g. here: http://pwcircuits.co.uk/microstrip-line-calculator/

https://resources.altium.com/p/fr4 says the dielectric constant of FR4 material is 3.3-4.8 (depends on weave style, resin content, and material composition).