Because your account is currently your phone number, so if they want to open the service to a limited number of power users, it makes sense to restrict it to folks who already are signal users, IMO.
(In fact, I do still expect public phone numbers to be the "default", i.e. encouraged, experience, because of its viral properties. This is also fine by me, as I want Signal to be used by as many people as possible.)
Do you have a recommendation on how they would prevent fraud and abuse w/o using a phone number while also maintaining the same level of low friction?