Hi Howard,
Having looked into it abit more now, I am thinking it is more this https://github.com/moodle/moodle/blob/MOODLE_36_STABLE/config-dist.php#L683-L688
As this would prevent settings being changed from the Web Interface.
It looks like what you pointed at changes the default value but doesn't prevent the value from being changed through the web interface moving forward.
Thanks
Alistair