WordPress logic forces WP_DEBUG to be defined to something, even if it's omitted it will be set to false in wp_initial_constants() during load.
However "background" (that is not when it is checked explicitly) function of WP_DEBUG is to be a flag for how PHP error reporting should be configured on runtime. That configuration is performed by wp_debug_mode() and at any point after that can be changed by your plugin's code if necessary.
