The point is that the TPM PIN prevents the attack if the system is powered off when the attacker obtains it.

If the TPM doesn't have a PIN, this attack works even if the attacker obtains the system when it's powered off. They can start the computer, proceed to the Windows logon screen (that they can't get past and that hence prevents them from exfiltrating data from the running system), then just reset the computer and perform this attack to obtain the encryption key. This obviously doesn't work if the PIN prevents Windows from ever even starting.