It is highly likely that this name refers to a creator of . SilverBullet is a popular web testing and automation tool often used in credential-stuffing circles to create "configs" that check the validity of account logins for services like ExpressVPN.
: Documentation on their proprietary, open-source VPN protocol designed for speed and security. EXPRSSS VPN by devil.svb
If you are looking for official technical documentation or research regarding ExpressVPN's security and architecture, you should refer to their verified publications: It is highly likely that this name refers to a creator of