The other part of the answer from the podcast was that it is technically possible due to the design of Signal for just about anyone (?) to look up whether or not a phone number has connected to the network, so in addition to the virality aspect they'd prefer to have this be more explicitly known by users rather than hidden but technically possible.
I don't remember the exact situation around the contact discovery aspect, whether it requires a mutual contact upload for discovery, or if is just generally available to anyone with your phone number, so please don't take this comment as verbatim on the technical details. This was just the gist of what he said.
