I think the intent is baby steps for those who aren't already privacy invested. Getting parents or tech illiterate to switch to Signal is a win. They weren't hiding their number anyway.

I currently have a Signal account tied to a sim not in my name. Getting a burner with cash is an option then park it. Or, if you're lucky enough to live near a payphone and can gain access to the number, you can activate a signal with a phone call.