You're mixing up the authentication and attestation parts of secure boot here. You can absolutely install Linux, run secure boot (e.g. to protect you against "

You're mixing up the authentication and attestation parts of secure boot here. You can absolutely install Linux, run secure boot (e.g. to protect you against "evil maid attack"), use your TPM to store your SSH keys, and live a happy and attestation-free life. You can also do other things, but if you don't want to, why would you?