As is stated in the documentation, Area2D objects just detect overlapping objects (another Area2D overlapping it or another body overlapping it) and are not simulated by the physics engine, all the movement of an Area2D needs to be defined by you.
Different from Physics Bodies (RigidBody2D and KinematicBody2D*), that detect collision and are simulated by the the physics engine. In other words, they need force vectors (intensity and direction) that are applied to them so that the physics engine move them for you.
Since gravity is a force/vector, it is only used by Physics Bodies, so it is not applied to an Area2D. As stated in the paragraph that you highlighted, Area2D objects can have a gravity property because they can pass that gravity to a Physic Body inside them. You might be familiar with the game VVVVV or Super Mario Galaxy where you have special areas that change the gravity (make it stronger or completely invert them, making you go upwards), an Area2D can create such an effect.
For a better explanation and examples, refer to this article from the documentation.
*You also have StaticBody2D, but they are not suppose to move.
