In order to establish Xfinity Mobile service, an SSN is mandatory. Requiring an SSN in order to start a mobile account is an industry wide practice used to curtail fraud schemes and not an act of discrimination. But before you could get Xfinity Mobile, a Comcast Xfinity residential service account would need to be created.
In order to get Xfinity mobile, when you are in the U.S., you would need to live in the Comcast Xfinity footprint and establish residential services first and foremost. Customers who obtain Comcast residential services and use an SSN to start the account do so in order to avoid a deposit.
Only after you have established residential services, would you be able to get Xfinity Mobile and then have the requirement of needing a valid SSN.