So it sounds like messages are associated with accounts, and accounts are linked to phone numbers, and those phone numbers are easily recoverable from a centralized database. If htat's accurate then it's not simply imperfect anonymity. It's not even pseudo-anonymous. It's about as not-anonymous as it gets, which is fine for a casual messaging/chat app where no anonymity is expected. But users should not be misled into thinking it's safe to use it for anything they wouldn't say to a government official's face.