The passphrase you set on a GPG private key only applies to keys stored on-disk in the keychain or exported from the keychain. Once you import one into a Yubike

The passphrase you set on a GPG private key only applies to keys stored on-disk in the keychain or exported from the keychain. Once you import one into a Yubikey or other OpenPGPCard device, there is no passphrase anymore, you're relying on the card itself to protect it against side-channel attacks, prevent unauthorized or insecure export of the key (Yubikeys don't allow this at all though), and authorize the use of it via the PIN code.