I hadn't realized before, the keyhole garden, at least in the African version, is also a kind of hugelkultur. In building the high raised bed, it has lots of browns at the bottom - wood, logs, in one case from the second link "the bones of two dead cows," etc.
The notch part is so that you can keep adding compostables to the center pile as well as watering through the center.
Does that mean you should treat it like a spiral garden - plant the most water loving stuff near the center and drought tolerant stuff around the outside? It seems like if you only water the center, the area right around that would get more of the water and nutrients. Might make the outer edge a good place to grow sage and other Mediterranean herbs, that don't need a lot of water or fertility.