Note that wp-config.php does not normally change after initial installation of WordPress, so if what they say is true, that would be the easiest place to add the code.
I’ve never heard of doing this before, and I’m not sure where you would add that – probably at the end. Try it to see if it works. Would be much simpler than a child theme. And if that suggestion is true, then you should also be able to add it using Weaver Xtreme Plus actions and filters exactly as is.
Plugins load before the theme, and that plugin has apparently defined a global variable that can be set from almost any point in a theme or other plugin, or even in the wp-config.php file which would load before almost anything else. Very unconventional approach to the “WordPress way” of doing things.