The rear wing has a pretty big gurney flap too.
Just checked the Tech Regs and there is an article:
3.10.2 Any bodywork behind a point lying 50mm forward of the rear wheel centre line which is more than 730mm
above the reference plane, and between
75mm and 355mm from the car centre line, must lie in an area
when viewed from the side of the car that is situated between the rear wheel centre line and a point
350mm behind it. When viewed from the side of the car, no longitudinal cross section may have more than
two closed sections in this area.
This allows a 150mm wide section that is not covered by the two-plane limit.
From MarioF1's images:
The area that isn't grey is where this may go.