Signal has spoken [1] at length about the issues of private contact discovery, and the many [2] solutions they've employed to minimize the amount of information that gets leaked. There's this intractable problem of requiring a social graph for easy discovery and for trust, and Signal's found one of the better solutions (though I'm sure we'll discover better ones yet).
Signal could allow using it without a phone number, and let people themselves choose between anonimyty, social graph and trust. Why didn't they allow it? Because they don't want anonymous users I assume.
But the issue in the parent post is about leaking information to the people you have in your contact list.
These are very different issues. And it looks like signal hasn't considered the second aspect and the implications.