The server doesn’t need to know or keep track of who’s sending a message to deliver it. If you don’t trust signal to not lie to the court about not collecting such metadata, I can’t convince you otherwise. But there’s a merit in designing your system so that such collection is as hard as possible.
Are you’re familiar with how singal’s servers work? Even I can think of a system where all messages are collected in a common pool before being distributed, the actual security researchers that made signal surely thought of something better.
How does FISA make it legal for singal to lie to a court about what information they have? Please enlighten me