It does know the recipient it doesn't know the sender. They call this "Sealed sender" and it is enabled by default for your friends (but you can change who gets this facility).
So instead of a message from Steve to tialaramex, it's just a message to tialaramex. Well, duh, of course tialaramex gets messages, why else have message software?
My Signal client prepared some "stamps" which are good for one message to me. It gave Steve (and all my friends, or maybe only Steve, or maybe everybody except one troll, Signal can't tell and doesn't want to know) some of the stamps when sending them other things, and so Signal just sees the message has a stamp on it, no need to know who sent it.
Your Signal client is at this moment logged into a Signal account on the Signal server. Sealed sender does nothing to protect against this - Signal knows that tialaramex is at that IP address.
The server fully aware where Steve is logged in from, and sees a message come from there to tialaramex. On top of even that: you then reply back, server sees a message going to Steve, going straight back to the IP address where it already knows he's logged in from.
Another thing people don't consider is that Signal's core server infra is hosted at AWS... so Amazon can also peek into both this network traffic and also dump out that it's your Signal account (ie. phone number) tied to that IP from the EC2 instance's memory.
So instead of a message from Steve to tialaramex, it's just a message to tialaramex. Well, duh, of course tialaramex gets messages, why else have message software?
My Signal client prepared some "stamps" which are good for one message to me. It gave Steve (and all my friends, or maybe only Steve, or maybe everybody except one troll, Signal can't tell and doesn't want to know) some of the stamps when sending them other things, and so Signal just sees the message has a stamp on it, no need to know who sent it.