SimpleStorage was the early model in Redfish for showing storage devices, and "Devices" in this resource do equate to drives. It's been superseded by the Storage resource for representing the more complete method of showing a storage subsystem, its controllers, devices, and volumes. SimpleStorage intentionally did not scale beyond direct attach storage since it predated any introduction of the fabric model.
It is okay to report both SimpleStorage and Storage simultaneously for the same storage subsystem. There may be existing clients that rely on SimpleStorage that do not understand the full Storage model. If you do support showing a storage subsystem as both SimpleStorage and Storage, there is a property in the Links object of each resource to step between the two views; it also allows newer clients understand that the two resources represent the same subsystem.