More precisely, "Messages in iCloud" is orthogonal to "iCloud Backup". If you only enable "Messages in iCloud" but disable "iCloud Backup" then according to tha

More precisely, "Messages in iCloud" is orthogonal to "iCloud Backup". If you only enable "Messages in iCloud" but disable "iCloud Backup" then according to that the messages will be stored in the cloud but still be E2E encrypted with a key that's not readable by Apple. If you disable "Messages in iCloud" but enable "iCloud Backup", then messages will be included directly in the backup, no separate key needed. If both are enabled, then messages are in a different cloud "bucket" than the backup but now will be readable with a key stored in the backup. So it's "iCloud Backup" specifically that needs to be disabled; simply disabling the Messages switch under iCloud doesn't affect this case.