Get and loop posts with all taxonomy terms

Your best option will be to get an array of term ids by using get_terms which belongs to the authors taxonomy and then using that as your array of terms in your tax_query

The following requires PHP5.4+

$term_ids = get_terms( 'authors', ['fields' => 'ids'] ); // Only get term ids 
$args = [
    'tax_query' => [
        [
            'taxonomy' => 'authors',
            'terms' => $term_ids,
        ]
    ],
    // Rest of your arguments
];