signal requires a phone number to sign up. a phone number could be used to trace your signal account back to you. so why do people, especially privacy enthusiasts and experts (like edward snowden), still use it and endorse it when it lacks anonymity in that sense? i get that people could use a voip number or something to sign up, but still.
This is a well known issue for a lot of people and signal has been promising to fix this for years. Using a phone # for verification reduces spam and bot accounts, but like you said it drastically reduces anonymity.
That said, anonymity != privacy. You could anonymously place a note on a car telling the owner to learn to park, but it's not private since anyone walking by could read it. Likewise you can send an encrypted signal message to another user who knows who you are, so it's private, but not anonymous.