If iCloud is not a backup utility, why does iTunes say "iCloud, Back up the most important data on your iPhone to iCloud"?

Apple does offer iCloud Backup for mobile devices. See: How to back up your iPhone, iPad, and iPod touch - Apple Support

However in addition to that, Apple offers multiple options for syncing data to iCloud such as iCloud Photos and iCloud Drive.

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers. It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see: https://support.apple.com/mac-backup

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

iCloud Drive FAQ - Apple Support

Other than the built-in backup function in iOS and iPadOS, other functions in iCloud synchronize data across devices. So iCloud photo is not a backup. It is a system to sync any and all changes on any device to all other devices. Similarly for Contacts, Notes, files in Pages or Numbers. Those files in iCloud Drive get synchronized across devices. Since that sync function includes deleting photos or files, those services are NOT an archival backup of files.

So if you use iCloud photo it is still recommended that you make and keep archived backups of your photos somewhere else -> Set up and use iCloud Photos - Apple Support (my underline emphasis added).

”Download copies of your photos and videos

When you turn on iCloud Photos, your photos and videos automatically upload to iCloud. They're not duplicated in your iCloud backup, so you should keep backup copies of your library. Use the steps below to download copies of your photos and videos to your Mac or PC. Or you can import your library to your Mac or PC.“

Depends on how iCloud is used. And what the context of the answer given was.

iPhones, iPads, and iPod Touches can be backed up to iCloud directly from within themselves. iTunes cannot itself backup to iCloud.

Mac computers, PCs, and other devices cannot be backed up to iCloud either.

Apple will suggest iCloud first, but an iTunes backup can still be performed if needed even if iCloud is being used to backup the iPhone already.