The enemies (Mob) are RigidBody2D's, so that their movement is automatically handled by the physics engine. The parameters (initial position, rotation, velocity) are set up in the function _on_MobTimer_timeout() in Main.gd.
With Area2D, you would have to handle the movement manually. For example, you could add the function _physics_process() to Mob.gd and do the movement there.
