It like theirs is few missing cases on overridingerror page :

1. PHP errors:

If you have PHPerrors like TypeError or Division by zero
Then you can Override theSymfony\Component\HttpKernel\Controller\ErrorController using the service.yml when in production env.
You will just have the exception to render your page but you still can use Twig.

2. Errors that happens before Twig was initialized

This can happen when you do not have a connection to your database for instance.
Then you need to set the template of HtmlErrorRenderer on your kernel :
HtmlErrorRenderer::setTemplate($this->getProjectDir().'/templates/error.html');

3. The PHP is not responding (503)

It might be good to mention that you should configure a pure html error page on your upper layers like nginx/apache/traffeik/varnish/...

Note : I'm not sure this the best way to solve this issues but I think the doc should mention that you do not catch 100% of errors withOverriding the Default Error Templates orOverriding the Default ErrorController.