the action admin_menu is called with one empty argument, then anonymous function can only be with 0 or 1 useless argument.
and then if you want to pass variables to this function, you can try that :
public function add_submenu_page( $title, $capability = 'administrator' ) {
$post_type = self::uglify( $this->post_type_name );
$GLOBALS["MyPlugin_menu"] = [
"post_type" => $post_type,
"menu_title" => self::beautify( $title ),
"menu_capability" => $capability,
"parent_slug" => 'edit.php?post_type=" . $post_type,
"menu_slug" => self::uglify( $post_type ) . "_' . 'menu_' . self::uglify( $title ),
];
add_action("admin_menu", function() {
add_submenu_page(
$GLOBALS["MyPlugin_menu"]["parent_slug"]
, $GLOBALS["MyPlugin_menu"]["menu_title"]
, $GLOBALS["MyPlugin_menu"]["menu_title"]
, $GLOBALS["MyPlugin_menu"]["menu_capability"]
, $GLOBALS["MyPlugin_menu"]["menu_slug"]
, function() {
echo 'The Page Actually Worked!';
}
);
});
}