So it depends on what you view as the primary functionality of this app. If the primary functionality is the syncing between multiple devices, then this reject

So it depends on what you view as the primary functionality of this app. If the primary functionality is the syncing between multiple devices, then this rejection is invalid and I understand the developer's dismay since an account would absolutely be necessary. If the primary functionality is "save this article to read later", then I can understand the reviewer's rejection. In theory, you could implement this software locally for single device, which wouldn't require any registration. And then you would only need an account if you wished to enable the "secondary" syncing functionality. I think this was the reviewer's frame of mind. So I think this is simply a case of miscommunication and misunderstanding that will be corrected shortly.