THAT was a mistake, and I apologize for it. It is entirely likely that Dale did not realize the thread had been locked. The "reply" button continues to work as normal for staff members, so it is all too easy to respond in a locked thread if they didn't notice it was locked to begin with.
I agree with you 100% that staff members should not post in locked threads in order to argue with someone that can't respond back in the same fashion.