The patterned panel inside the medallion is not upholstered onto the diwan. It is made as a separate piece and dropped into the carved ring from behind, and there are good reasons for doing it the harder way.
The first is placement. A floral pattern inside an oval only works if the design is centred in it - a bouquet that sits low or off to one side looks like a mistake, however good the fabric is. Made as its own panel on the bench, the cloth can be moved around on the former until the pattern is exactly where it should be, checked, and only then fixed. Try to do that in place, with the frame already carved and gilded around the opening, and you are working blind.
The second is the future. Fabric is the part of any diwan that ages first. Because the medallion panel is an independent drum, it can be lifted out and recovered on its own years later - a new pattern, or the same one refreshed - without a tack being pulled anywhere near the silvered carving.
The panel is held from behind with turn fixings, so removing it takes minutes and leaves no mark.