Interesting. I wonder what design they'll come up with. The thread links to a tweet from Moxie from a few months ago, which (along with some other tweets in the thread) is interesting to think about:

https://twitter.com/moxie/status/1174047779267604480

In theory they could keep using the native contact list and just stuff Signal usernames in there; iOS does have the APIs to do that, and I'd assume Android too.