Check out this ramp. Filmic is shown, but it’s disabled; the marked area (visible due to ‘display sample areas on image’ in the global colour picker) shows mid-grey, or LAB 50%:
Take a snapshot of this starting state.
Hover over filmic and enable labels. You’ll see that the orange dot (18%) is mapped to 18% on the y axis:
Compare with snapshot: mid-grey is unchanged:
Make some crazy adjustments to filmic: mid-grey remains mapped to mid-grey.
This remains true even if the mapping is completely bonkers, like this:
