After deleting the default code and replacing it, the customizer is updated with the new variables as normal.
However, when F5 or F6 is pressed the original variables come back (title and debug).
The customizer reverts back to its proper state after the file is edited or the render button is pressed.
Tested on firefox with this script:
$fn = $preview ? 16 : 128;
x = 4.2;
cube(x);
sphere(x);