First digits of the credit card tells you what card it is and who is the issuer. Detecting privacy.com is just a matter of knowing their ranges. They also them

First digits of the credit card tells you what card it is and who is the issuer. Detecting privacy.com is just a matter of knowing their ranges. They also themselves will prevent you from using those cards to break ToS if they can detect it (i.e. multiple sign-ups for known service).