Looks like there is a bug in the software. 
I closed a thread and got page where "reply" button changed to "closed". But if/when you click that button i.e. "closed", it takes you to the form where posting to the same thread is permissible, even though thread is already closed.
I closed a thread and got page where "reply" button changed to "closed". But if/when you click that button i.e. "closed", it takes you to the form where posting to the same thread is permissible, even though thread is already closed.