The company hosting that URL offers a product for syncing between various clouds, I haven't used it but it does exist. https://www.multcloud.com/download
So I guess this is another one of those things that just isn't true. Go figure.
Not really. For Dropbox, Nextcloud, Photo Sync, et c to upload your photos, the app needs to remain open; it can't upload in the background for more than a minute or so.
This means that only iCloud can do real background syncing. If you want to upload a full camera roll to a non-iCloud service, you're in for a world of frustration. You'll have to disable screenlock and put the phone on a power cord and leave it open with the app up for hours and hours. Of course iCloud doesn't have such limitations.
No non-Apple app is allowed to do background sync, no matter what you install. They have put iCloud Drive and iCloud Photos in a privileged position.
Syncing is a different story, let's see how this holds up:
If you want to sync your iPhone's photos to Dropbox, you give Dropbox permission to access the Photos library and it syncs. https://www.multcloud.com/tutorials/sync-iphone-photos-to-dr...
The company hosting that URL offers a product for syncing between various clouds, I haven't used it but it does exist. https://www.multcloud.com/download
So I guess this is another one of those things that just isn't true. Go figure.