Joe B
Pre-takeoff checklist
The trainees missed that and, thinking the 172 had left the runway, cleared the RJ for takeoff.
Must be a trainee journalist, too. You can listen to the whole deal in the archives on liveatc.net. It sounds to me like the controller cleared the RJ for takeoff before the Cessna announced a change in runway exit. You can almost hear the controller gulp as he tells the Cessna to exit the runway "no delay".