I didn't look but it seems to me that it would be due to the context sensitive nature of Nudge. As in the Nudge feature does different things in different views, when different material is selected, etc.
So the "Nudge" bindings will likely be in the shortcut Preferences and documentation but not all the things it does in all contexts. As for documentation the Nudge actions would likely be described in sections about specific views, events, etc (like what nudge will do to a clip in the track view vs. what it will do to a note in the PRV, based on what's selected, etc).
Also if the Nudge keys aren't listed in the shortcuts Prefs maybe they are permanently bound specifically because of that transient nature.
Dunno.