Boost already overrides the core course renderer and has the line
class course_renderer extends \core_course_renderer {
Other themes that then extend Boost look something like
namespace theme_waxed\output\core;
defined('MOODLE_INTERNAL') || die();
use moodle_url;
require_once($CFG->dirroot . '/course/renderer.php');
class course_renderer extends \theme_boost\output\core\course_renderer {
See https://gitlab.com/ro-mdl-themes/moodle-theme_waxed/blob/master/classes/output/core/course_renderer.php#L26-40 for example
You also say you are extending Snap, but you only have Boost as a parent - are you extending Snap (using it as a parent) or have you cloned it and customising it?
Richard