Crew miss-programs autothrottle such that it commands a climb. Crew doesn't want to climb and so overrides and disconnects the autothrottle. Crew then expects the disconnected autothrottle to maintain airspeed during final.
And somehow that's the fault of the autothrottle?
I think the crew is saying that they did not expect the autothrottle to disconnect when they disconnected the autopilot. In and of itself, that doesn't seem unreasonable, no?