Building a completely empty application with just the DI component loaded, running it barebones in the SSCCE:

$builder = new ContainerBuilder();
$builder->compile();

$dumper = new PhpDumper($builder);
echo $dumper->dump();

This terminates with:

Uncaught error: Class 'Symfony\Component\Config\Resource\ClassExistenceResource' not found in <...>/ContainerBuilder.php:330

As Config was never loaded. Likely the DI component should just explicitly depend on the Config component itself these days.