CM has confirmed that this is the case. 750 reply limit is locked for every board. He asked if we would like to have that option and I said it'd be good to have, so it may be a possible option soon.
Every time a post is made in a thread, the thread is bumped to the top of the index/catalog on the board. A bump limit only prevents the thread from being bumped to the top of the catalog every time a post is made inside it, but still allows replies. A reply limit is literally a limit to the amount of replies permitted per thread.