One statement is not related to the other here.

Getting and maintaining an active phone number privately is indeed quite hard, partially by governmental design.

Signal only requires occasional/rare proof of control of the registered phone number. It also has very little visible data the provider can access on your account, even if they had a reason to assist in breaking your privacy by look it up from the phone number. Without Signal foundation direct support, the phone number linkage to your Signal account is completely opt in by you only.

So in terms of privacy, Signal is actually very good about the phone number and leaves it mostly to you how public you want to be about it. They're primarily using it as a finite controlled resource to limit how easy it is for people to spin up arbitrary new accounts. Other projects might use some cryptocurrency junk that effectively equates to paying for accounts, but Signal uses what you probably already have.