Yes, heard that some years ago as well (SOU was mentioned). Talk back then was that the airport tried to talk them into using MGL in order to save DUS limited runway capacity for larger aircraft (and probably refused to support a service from DUS).
DUS now retired long-term CEO was famous for bringing the blood pressure of Lufthansa's DUS chieftain up by always complaining about the use of "Lufthansa Challengers" on many of Lufthansa's DUS services - he was referring to the Canadair Regionaljets that, as he was convinced, were a waste of runway capacity.....