Yes, how I see it working is a bit like around 1 minute into that video. You'll be able to draw all the different types of CityGen3D objects out in 2D (such MapSurfaces and MapRoads). Then any generator settings and functionality would apply to these just as they do for those created by OpenStreetMap at the moment. Hypothetically if you draw a mesh even now, say using Pro Builder, and assign a MapSurface component to it, the generator would see that just as if it had been data from OSM. But obviously its really unintuitive at the moment. So this feature will be about providing a user friendly way of being able to draw the 2D maps using a toolbar and clicking out shapes and lines in Scene view (top-down) to make your residental zones, roads and whatever else.