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.
-Stoic Joker
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.