“some apps have a core use case that requires broad access to files on a device, but can't access them efficiently using the privacy-friendly storage best practices. Android provides a special app access called all-files access for these situations.”
“For example… anti-virus apps… file manager apps, backup and restore apps, and document management apps“
https://developer.android.com/training/data-storage/manage-a...
NextCloud provides backup, restore, and document management.
I guess you may know more about this than the official android docs, but maybe you are just being blinkered by some incorrect assumption?
Maybe we would be best to frame this another way. What permissions do NextCloud need to be able to replicate “Google One”? The complete replacement of Google drive (the service, not the app - you are consistently conflating the two) is what they aim to be. Is that something that can be done with SAF, or are Google using elevated permissions to lock competitors out?