ThreadCountersListener.php

<?php

class ThreadCountersListener implements EventSubscriberInterface
{
    protected $commentManager;

    public function __construct(CommentManagerInterface $commentManager)
    {
        $this->commentManager = $commentManager;
    }

    public function onCommentPersist(CommentEvent $event)
    {
        $comment = $event->getComment();
        $thread = $comment->getThread();

        $count = 0;
        foreach ($this->commentManager->findCommentsByThread($thread) as $comment) {
            var_dump($comment->getState());
            if ($comment::STATE_VISIBLE == $comment->getState()) {
                $count++;
            }
        }

        $thread = $comment->getThread();
        $thread->setNumComments($count);
        $thread->setLastCommentAt($comment->getCreatedAt());
    }

    public static function getSubscribedEvents()
    {
        return array(Events::COMMENT_POST_PERSIST => 'onCommentPersist');
    }
}

You may try this

public function onCommentPersist(CommentEvent $event)
{
    $comment = $event->getComment();
    $thread = $comment->getThread();

    if (!$this->commentManager->isNewComment($comment)) {
        // if was invisible and now visible - increment counter
        if ( $comment->getState() == $comment::STATE_VISIBLE && $comment->getPreviousState() != $comment::STATE_VISIBLE ) {
            $thread->incrementNumComments(1);
        }
        // if was visible and now invisible - decrement counter
        elseif ( $comment->getState() != $comment::STATE_VISIBLE && $comment->getPreviousState() == $comment::STATE_VISIBLE ) {
            $thread->incrementNumComments(-1);
        }
        return;
    }

    if($comment->getState() === $comment::STATE_VISIBLE) {
        $thread->incrementNumComments(1);
    }
    $thread->setLastCommentAt($comment->getCreatedAt());
}