It is not about that. The pixel has a TPM chip (Trusted Protection Module). This is similar to how secure boot works in desktop computers. It is a special external chip that has a secret internal cryptographic key that can never be accessed by anyone. This chip can be used to create secured communications between devices. This is how it is possible to do over the air updates securely and how the device’s security can be checked with a special app and an external device like an old Graphene phone. All files on the device can be hashed with the secret key to determine of they have been changed. Other phones do not include a TPM chip and this is the primary reason they cannot be supported directly by Graphene.