It doesn't have to have a green sign to be called a motorway, more properly a controlled-access highway. As long as a road has a central divider or median, has no intersections, has no pedestrian crossings, and has no curbside sidewalks, it can be called a motorway or freeway. Now, Lung Cheung Road does have sidewalks, however, they are separated from the road, that is, pedestrians cannot accidentally step into the street, so it is still a motorway.

https://en.wikipedia.org/wiki/MEMU

Fare systems or speed does not matter.

The Airport Express is not intercity, therefore, it is a subway.