There was also a screw or two at the front of the side skirt.
If you had an interior, you’d have to take it all out to get down to here to remove the rear bumper. There are various studs that stick through into the trunk and you remove the nuts from there. You also need to remove the headlights to access some of the retainers that sit under there. And, finally, at the top center part of the trunk you will need to remove a factory trim piece to get the last of the retainers.
My car, fortunately, was missing most of that hardware, since it was attached well enough… I think.
On my car, two of the strips were riveted to the bumper, and the other two just fell out. Don’t lose them. You need these strips to go through the holes in the bumper and then into the holes in the chassis, and the nuts go on from inside the trunk. This pulls the studs into the trunk which pulls the rear bumper onto the car.