Is there any reason why they couldn't just use a burner phone for the account signup and then toss it? If the phone isn't required for subsequent logins then it really isn't accomplishing anything ... It's just security theater.
I've had unwelcome experiences with Google where, down the line, they ask for verification of some piece of information in order to validate the security of the account.
I wouldn't put it past them at some point in the future, even if you don't have to use it to log in all the time, they did something to validate it.