At the moment, the author can reply to his own topic that is for approval, but the moderators or admins or any other group that have permissions to read an unapproved topic cannot reply to it...
You may wonder "Why allow replies to the topic if it is not approved?"
Because a moderator or an admin, that have permissions to see an unapproved topic, might want to reply to it for some reason and have a hidden discussion in it.
In my case the reason is that all topics should follow some guidelines, and if the user doesnt create the topic properly I need to have an discussion about it.
There are many reasons for having a discussion on an unapproved topic.
Any forum that I used had this, and I beleive phpBB 3.0 had this too... so it must be a bug on 3.3 branch at least?