>I suspect people want their entire photo folders mirrored into Nextcloud from the device
That isn't remotely the contention, nor do photos even qualify for this as they use a different API. Further, the reason this company gives for refusing to use the obviously more suitable structured storage API is that they don't want their files -- presumably mirrored from the cloud storage -- visible to other apps. Their complaint is technical nonsense and doesn't pass an ounce of scrutiny.
The argument by this company is nonsensical, and their argument seems to be "we did it this way before and we don't want to change". Firstly they can have their own app storage without granting access to any other app, and they can go through a system UI process to get access to additional folders (for instance "I want to back up my WhatsApp folder to this cloud provider"). They argue against the latter because they seem to think it somehow reveals the former, but that isn't the case whatsoever.
[1] - Well it's a bug in the Nextcloud product where they seem to just ignore that the instance lacks a permission