Why does the Offloaded tab say "Reconnect your fast drive to restore"?

Short answer: Offloaded apps live on your fast drive, which isn't connected. Plug the drive in and Restore becomes available again.

Your offloaded apps are stored on your fast drive, which isn't connected right now. Plug it in and the Restore button becomes available again.

The list stays visible while the drive is away, so you can still see which apps are archived and how much space they use.