Based on that screenshot, it looks like this feature is tied to the primary iCloud on a device, which kinda sucks.
My wife and I share the same iCloud account for contacts, calendar, and photos. I use that same iCloud account for iMessages, while she has her own. I was hoping this feature would allow me to backup my iMessages to the primary iCloud account and allow her to backup to her account.
Can anyone confirm if there is a setting in the Messages app itself that references the backup of non-primary-iCloud account messages?