Use this snippet instead:
add_filter( 'generate_post_date_output', function( $output, $time_string ) {
// Set Published Date string
$label = 'Published: ';
$time_string = '<span class="date-label">%5$s</span><time class="entry-date published" datetime="%1$s" itemprop="datePublished">%2$s</time>';
// Compare updated and published dates
$updated_time = get_the_modified_time( 'U' );
$published_time = get_the_time( 'U' ) + 86400;
// If updated then update Date string
if ( $updated_time > $published_time ) {
$time_string = '<span class="date-label">%5$s</span><time class="entry-date updated-date" datetime="%3$s" itemprop="dateModified">%4$s</time>';
$label = 'Last Updated: ';
}
$time_string = sprintf( $time_string,
esc_attr( get_the_date( 'c' ) ),
esc_html( get_the_date() ),
esc_attr( get_the_modified_date( 'c' ) ),
esc_html( get_the_modified_date() ),
$label
);
return sprintf( '<span class="posted-on">%s</span> ',
$time_string
);
}, 10, 2 );
And add this CSS:
.entry-meta .author-name,
.entry-meta .date-label {
font-weight: 700;
}