A PIN and passphrase fulfill the same goal: adding an additional layer that is required to use the keys.

They can both be created and manipulated the same ways, whether it be creating via entropy grids, joining 2 separate passwords together, etc.

If your HWW started treating the passphrase like a 2nd PIN today you wouldn’t even notice.

Except a passphrase is significantly harder to change than a PIN.

If someone sees your passphrase when using your HWW, that weakens the security of your paper backup. But not if they see your PIN.