Signal doesn't know who you are. A number don't reveal your identity, and, usually, you should be just a gov. entities to discover that. Does matter what you call it: Signal is for privacy (they have your number, but they don't know who you are, who you write and what you write), not for (full) anonymity (they don't have any information, including number, on you).
Anyway, they're implementing the username too.
Honestly, I think the problem here is that people have the habit to use the wrong app for a specific use. Signal is, conceptually, similar to WhatsApp, Built for discussing with people you already know (for WA is stated on the website too "Friends and family"), not for any stranger people in the world. So, it shouldn't be used for that, app/protocol like Matrix, Threema and Telegram fit better that purpose