I believe that’s correct, and you do need the root server to be subscribed to the SWD site for the SWD server installation tasks to become relevant for it.
:edit: technically I think both statements are correct. You could probably register the SWDDownloadPlugin independently of the Server Plugin Service and the Upload Maintenance Service, but you’ll need all of them to use the dashboards anyway.
The Server Plugin and Upload Maintenance services deal more with using the Console to get SWD packages into the server; and the SWDDownloadPlugin deals more with how clients and relays retrieve your packages during action executions.