The only reason I can see for disconnecting the torque convertor is MAYBE the torque convertor clutch locked to the impeller. But even then, there are no clutches applied which would cause the engine to not turn. The input shaft is connected to the turbine inside the torque convertor and has no physical connection to the impeller (case) unless the TCC were locked and somehow a clutch pack were remaining applied. This certainly sounds like an engine failure, in which case unbolting the torque convertor will help out to pull the engine.