See sibling thread, we're being duplicated; the point of this OAuth flow is to sign in on a different device, using the trusted one. That different device might be a legitimate TV with a TPM and cryptographic attestations that it truly belong to John Doe, there is still no way for your iPhone and Apple to check whether you meant to sign in to John Doe's TV or if they are a scammer and sent you the (legitimate) sign-up link over email.