I talked to Moxie about this 20 years ago at DefCon and he shrugged his shoulders and said "well... it's better than the alternative." He has a point. Signal is probably better than Facebook Messenger or SMS. Maybe there's a market for something better.
I have no idea if that was true 20 years ago, but it's not true now. XMPP doesn't have this problem; your host instance knows your IP but you can connect via Tor.
I remember listening to his talks and had some respect for him. He could defeat any argument about any perceived security regarding any facet of tech. Not so much any more. He knows as well as I do anything on a phone can never be secure. I get why he did it. That little boat needed an upgrade and I would do it too. Of course this topic evokes some serious psychological responses in most people. Wait for it.
Is there any reason they didn't use email? It seems like something that would have been easier to keep some anonymity., while still allowing the person to authenticate.