They are really two different functions. We don’t consider sync a solid backup.
For example, it is optional to sync. You can have all your data local to the device. And it is also possible to use Dropbox for sync, rather than iCloud. We also allow devices to remove the sync data.
In general, we think it is better to separate the two functions. If we didn’t, there would be moments when you would be susceptible to data loss Eg.
You turn off the cloud sync, and choose to remove the cloud sync data
Agenda now starts to backup your data to iCloud. But this can take a while, and if something were to happen now, your data would be lost.
To add to @drewmccormack comments, of course if you find those risk acceptable and are confident you have everything in iCloud (for example because you see your notes appear on your other devices), and you are willing to risk loosing notes that have not yet made it into the cloud as Drew highlights, then you could choose to not include Agenda’s data in your iCloud backups. But again, not something we would recommend.