Earlier quoted context omitted.
Using a telephone number is impractical? That's our identity in the third world. I think they are focused on the developing world in this.
What if the user is on a WiFi-only tablet, or laptop, or desktop PC? What if the user is at an Internet cafe? What if the phone user has no cellular signal but has WiFi access? Clearly it would be a mistake to limit this to just phone users.
I agree with your point that in some countries people don't use Email and that's the face in China as well. But people use phone numbers just for register other services, as the online identity. We don't use SMS, and we prefer WeChat Audio over phone calls when it's available.