Yeah that's...... not really a "diffuser" either. Maybe by definition, but in order to be a functional diffuser, it needs to reach under the vehicle and direct airflow. Yours is probably directing airflow, but not correctly.
This is a more accurate depiction of a rear diffuser.
In today's automotive design, most cars have simulated diffusers because a diffuser is typically seen on sports/racing vehicles and if you make a car look sporty, you'll sell more of them.