Source

#39792 (get_the_comments_pagination should accept the list type argument) – WordPress Trac

get_the_comments_pagination uses paginate_links, which allows the following type arguments:
‘plain’, ‘array’ and ‘list’. Default is ‘plain’.

But compared to get_the_posts_pagination that changes ‘array’ to ‘plain’ but allows the ‘list’ type, get_the_comments_pagination always sets it to ‘plain’.

Comments:

// Make sure we get plain links, so we get a string we can work with.
$args['type'] = 'plain';

https://core.trac.wordpress.org/browser/tags/4.7/src/wp-includes/link-template.php#L2868

Posts:

// Make sure we get a string back. Plain is the next best thing.
if ( isset( $args['type'] ) && 'array' == $args['type'] ) {
    $args['type'] = 'plain';
}

https://core.trac.wordpress.org/browser/tags/4.7/src/wp-includes/link-template.php#L2527