All these platforms expose a kind of data repository with multiple users that is designed to be used as an app backend. The permissions in a "headless CMS" will by default be designed under the assumption that people will be writing articles that will all be shared between/accessible to all users whereas Firebase/Supabase/Appwrite will assume that each user has their own private data, but a lot of the details are in how you configure them for your use case and perhaps they aren't really that different when you think about it.