The "treatment as usual" control group was indeed treatment as usual which meant that some study participants were in on-going therapy and some weren't, some were on maintainence medication and some weren't, some were getting no treatment at all since they were in remission.

On your second question, I believe that there has been some research one the effectiveness of maintainence medication for preventing relapse in depression but I don't remember the details. I'll see if I can find it.