First, a sync to any cloud is NOT a backup. Please review the DEVONthink manual. The purpose of the sync is to make the databases available to other devices. When you attach to your iCloud account with another device and see the sync files (Menu: Preferences/Sync)?
I use Dropbox, not iCloud, so I can’t directly help you. I did a search on this forum and found Can't see remote iCloud databases
But: don’t rely on the sync file as a “back-up” as you suggest in your posting.