You're right that it's a global requirement, and wrong about everything else. It's a global requirement so it depowers Android applications, making them store as much data as possible in the /data/data folder that users can't access, and user-installed apps can't access, but you know what can access it? That's right, Google sync software and Google backup software, installed and running as root. Now, why do you think they put in that requirement. It's not enough to just tell us that they did. If there's no actual security benefit, then that answer is not good enough. They aren't going after Nextcloud here. They're going after a bunch of different apps, of which Nextcloud is only one. They've taken similar steps to reduce access to users' data through any mechanism other than OEM-installed system apps, for example deprecating ADB backup without replacement.
But maybe this is Nextcloud's problem because they should be using some more secure method. Let's see what other applications use. There are convenient privacy reports that show the permissions apps use. Dropbox has READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE, the ones that implement access to all files. OneDrive, Box.com, Amazon Drive (write only for some reason), all have these permissions. But maybe these are just all lazy companies. What does Google use? Google Drive uses exactly the same permissions to do it.
I don't think Google is targeting Nextcloud here. I think Google is trying to target everybody, but if they did the same thing to the big players like Microsoft or Dropbox that they're doing to the small ones, they might get some real legal attention. But if you think this is totally normal, you can explain why Microsoft, who has thousands of programmers at their disposal and updated their OneDrive app lots of times since Android 11 was released, gets to use these.