"Should they not also disconnect STS and therefore MCAS ? during such a situation ?"
of course, but initially only the AP/AT would be disconnected in response to the airspeed disagree, the auto/main trim switch selection would come when you recognise the trim runaway, assuming you are not distracted by airspeed limit alerts and tactile warnings, ironically it would appear that the STS/MCAS is only active once the AP is disconnected.
"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"
agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.
Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.
The point is, most pilots would assume that disconnecting the AP/AT would give you full manual control of the aircraft.
Clearly it doesn't.
Last edited by hec7or; 11th Nov 2018 at 19:44.