You dont need a redundant server, but i believe that is what the intention of the Backup URL. If you want to use that backup functionality, then you would need a backup server. And yes, that would require a new server and a new License to run the EMS.
If you don’t want to run a second server, you can set that Backup URL to a second stream on your single server (even to like a file or something): rtmp://YourEMSIP/vod/mp4:myfile.mp4