The question doesn't say maximum drag for that configuration any more than it says maximum thrust for a particular throttle setting.
You have assumed that the parameters of the question allow you to maximise thrust with the throttle setting. Both configuration and airspeed change drag, why assume one and not the other? My point is that you hit max speed when max thrust equals drag, not maximum drag. You can't increase the thrust in that condition, but you can increase the drag. You will actually get a higher maximum speed if you can reduce the drag.