Design choice should be "no need to trust". If not, nefarious motivations should be assumed, unless proven otherwise.

I have evidence. “Moxie” is simply not a person I would trust, for various reasons.

By a random ID as others [1] do?

[1] Threema

MAC address, randomised by the pocket-sized computer owner as needed. Perhaps laypersons are less able to distinguish computer networks from telephone networks than they were 20-30 years ago. Signal relies on a computer network. Signal runs on "personal computers" that are bundled with "phones", but not on phones that lack personal computers. Perhaps one day we will reach the point where "VoIP" becomes meaningless because all voice will be carried over IP.

Why do you need to identify your phone to use an online messaging service?

With a username and password