Not so silly a question:
In common with other INS equipped a/c with electromechanical HSIs, the HSI is selectable to "Rad" or "INS" modes.
In "Rad" it works in the manner you are probably used to, and referenced to the appropriate compass. In "INS" mode it is referenced to True, the CDI is referenced to the desired track as required in the INS, and the beam-bar indicates linear deviation from track. (If you look you should also see a small "lin" flag on the HSI).
Very similar to the 747 and, I am sure, other types which first flew in the 70s.