The diagram could be either, but isn't a perfect match for either. Perhaps a hybrid is possible. The 4-level right-angle construction (stair step) is weird.
Zone plates work great for radio waves. I don't know why they aren't used more than parabolic dishes, since they're easier to manufacture and mount. Perhaps the gain is smaller for a given diameter?
There are two dimensions to focus, and they don't have to be done the same way. For example, the trough antenna is parabolic in one direction but uses something else in the other direction. Viewed from along the resulting beam, it is a rectangle. Manufacture the reflector involves a simple 1-dimensional bend of the metal.
My guess would be twofold: adding delay if the source is not straight though enough to screw communication or diffusing the image too much if your focal point is not a point but a sensor.
https://en.wikipedia.org/wiki/Zone_plate
The diagram could be either, but isn't a perfect match for either. Perhaps a hybrid is possible. The 4-level right-angle construction (stair step) is weird.