Originally Posted by LarryJ

The procedure is not the "MCAS deactivation procedure". It is the "runaway stabilizer" procedure. A variety of systems, and failures, can result in a runaway stabilizer; MCAS is only one of them. Because an unchecked runaway stabilizer can quickly lead to a loss of control, you don't waste time trying to diagnose the reason for the runaway. If you have a runaway, you disable the electric stab trim which stops it. Let the mechanics figure out the cause of the runaway after you land. The runaway stabilizer procedure is the correct actions regardless of the underlying cause of the runaway. Since the autopilot also operates the trim (when it is on), the procedure has you disconnect the autopilot as part of the procedure because a failure in the autopilot could potentially cause a runaway. We're talking here specifically about a runaway caused by MCAS so, in that case, the autopilot must already be off by definition