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.