I think you're incorrect. True altitude relates to the error in the presumed height of the air column from the surface to the altimeter. Doesn't matter what setting is set on the subscale.
Even though QNH is a longer column - going all the way to sea level instead of stopping at some elevated ground reference point as with QFE - the portion from the surface down to sea level has no temperature error.
This is because there isn't a non-standard column of air from the surface to S.L., just solid rock etc so the assumed height / pressure change relationship doesn't have a somewhat elastic air column to account for.