Author here. Amusingly I tried that once with an Android VM; it accepted the input and tried to proceed, then failed with an unexpected error, implying the developer of the application hadn't anticipated this. The account entered some half-created state in which attempts to login with it on the web demanded a phone number.

Interestingly I am also now locked out of an old Gmail account I have had since 2005 in the same way; any attempt to merely login to it yields a demand for a phone number. There is no phone number filed for that account, so this cannot possibly be for "security". It is literally an attempt to extort a phone number out of me if I want access to my own data. One suspects this is probably against GDPR, of course...