Trying to find some explanation about the city icon on the world map, I found those topics:
Why does the game no longer show an icon for Photogrammetry cities on the map?
Unlisted cities with photogrammetry
World Map cities
So maybe yes, the intention of the city icon is to identify photogrammetric cities on the world map. Looks like it was working at release and something was broken at some point.
Maybe that information comes in the SDK documentation, if someone know where to look and confirm it would be helpful to confirm the logic.