TPMs typically won't give out a secret unless all of the software which has been loaded and executed was "measured" and found to be unmodified from when the secret was stored. So you couldn't simply stick your own hard disk or USB disk into the computer and then ask the TPM for the secret: the running software wouldn't match, and the TPM would refuse.

This is a problem because the "approved" software has this strange vulnerability to get into the rescue shell, right at the point where the TPM would be happy to give you the secret because the software is unmodified.