Correct. But you do know the passphrase, and, from the viewpoint of Secure Boot and the locked-down kernel, you (the legitimate owner) are also the attacker who

Correct. But you do know the passphrase, and, from the viewpoint of Secure Boot and the locked-down kernel, you (the legitimate owner) are also the attacker who tries to run some unapproved kernel-mode code and will stop at nothing in order to do that.