OK, I don't know much about the prune option. I never use it.

But I do know, by direct observation, that if you delete a post by a user, the user's post count goes down by one. I have seen this with my own two eyes. Last time was a few days ago...a user had double-posted. His post count was 4360. I deleted one of the two messages, and after that, the post count in the remaining thread said 4359. This is indisputable.

So regardless of pruning, a user's post count does indeed go down if any of the user's posts are deleted directly, and presumably also is an entire thread is deleted, too. So, it is clear the the post count does not reflect how many times the person has ever submitted a message.