On my iPhone in
Settings>iCloud>iCloud Backup
I had 'Back up Over Cellular' disabled. Underneath this toggle it seems to imply that it will only sync over Wi-Fi if this is off. However, it allowed me to backup anyway. I thought it might error out eventually, but it process completely and my device was then listed as a backup in the list at the bottom with the size of the backup.
Is there another setting somewhere that overrides this?
Am I misunderstanding the footnote?