Just a thought, but I would assume that the default action is used if the Field Scribe is the last friendly model on the board. Guess they would just make a run for it…
Re-reading the M action, I would assume they would always take a careful M toward their objective if they are the last model on the board for their side. The 0-1 and 2+ are aura distances, not number of models
The intention of the default action is for when a model is unable to perform its objective. Normally the default is [A]ttack but the Field Scribe performs [M]ove instead. I shall add that to the errata as that should be in the rules.