"Major contradiction" how? It's simply admitting a limitation in their reality and working around it. It's pretty reduced from what it used to be, that would be that the phone number would be known to all chat participants. Do consider that it took some guy spending years in jail to learn enough of the telecom system that he might (not even "will", but "might") be able to spin up his own mobilecom that only requires a ZIP code to sign up.

Signal is not 100% perfect, they're barely reaching 98% perfect, but. Fantastical (fanatical) absolutism in goals helps no one make step progress.