[dm33tri]

"The social graph" (your phone book) is most likely already uploaded somewhere by third-party app or even Google/Apple themselves. Using separate contact list (even uploaded to some server) seems more secure to me than using your phone's one.

The phone number is much more valuable to any authority than other metadata because they are more likely to have access to cell service than to messenger services.

In the context of the post, signal is much more vunerable than even basic things like email or web chats because police can effortlessly identify anyone in the group chat with a single request to cell company.