Supposing the scenario as presented, this group was not the entire population of the city, but more likely the loyalists living in and around the palace. Mesoamerican housing patterns show a definite clustering of homes into kin-based units. Thus it would not be surprising for Noah to be able to alert kinsmen and have them flee in concern, coming from one of these kin-based compounds.
The Lamanite attacking army would therefore face two options, chase the fleeing band that included the King, or turn and attack a city that would have been defended by Gideon and his men, as well as all who would have remained in the town. It is not hard to see the easy choice made by the Lamanites. If the group fleeing could be seen well at all, their dress would certainly mark them as high status, and the unorganized flight would make them a much easier target than organized resistance around a probably fortified location.