Hey Andre,
The count should be updated to the user immediatly after posting (on the article page - we update it with javascript, so that you immediatly have the feedback that your comment was posted).
But we can't do anything about the cache. We cannot delete a single item cache, we have to remove all the cache generated for all articles and if I do this then you are going to run into performance issues when users start posting all the time - you basically would end up without cache all the time.
So you'll have to live with a wrong comment count for 15mins untill you cache expires.
Regards,
Daniel