Yes, their blog posts are really good. Read about the sealed sender, too! And then realise that even with sealed sender, if you send from your home IP, then you are identifiable. So you should send from a reasonably anonymous IP (w.r.t. your threat model)
Can someone explain how this is possible? How does my phone know when to notify me that a friend has joined signal?
Edit: apparently it's "private contact discovery", I need to give this a proper read later.
https://signal.org/blog/private-contact-discovery/