In my opinion, Signal isn’t trying to be a Matrix alternative. Anonymity between users isn’t their objective, they mostly want to be a none profit alternative to WhatsApp, Messenger, etc… with strong E2E encryption. Phone numbers is ultimately the best way to discover new users as soon as you install the app.
These days the standard is to create an API Doc out of a OpenAPI document generated from the code itself. Someone will probably contribute to it at some point.