Yes, a reply is a post because you are doing regular replies on threads that means you're a regular user of the forum and you're taking part in posting. It will be count as a post for a signature campaign only but to post a new thread you will need to create a new topic like the one you created. The signature campaigns require good quality posts with minimum of 75 words or above and those posts can be either new threads or replies to existing threads. I hope that now you better understand the topic you asked about.