Can you just disagree with the design choice rather than implying, without evidence, nefarious motivations?

Ongoing work is being done to enable Signal accounts without requiring a phone number (there's been a number of changes to the protocol to support this in the last 6-12 months), it's just not done yet.

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.

How would you identify your phone otherwise?

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?

By a random ID as others [1] do?

[1] Threema

With a username and password