Using hook_node_submit() from a custom module
hook_node_submit() will only be invoked for the module implementing the associated content type. If module A defines the content type, module B cannot implement hook_node_submit() on module A's behalf, unless it specifically calls the function module_B_node_submit().

If you want to hook into the validation process of a node form you can use the standard hook_form_alter(), e.g.

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'article_node_form') {
    $form['#validate'][] = 'MYMODULE_node_form_validate';
  }
}

function MYMODULE_node_form_validate($form, &amp;$form_state) {
  // Logic here...
}