The flights you saw are sold under the HR designator so that they are available on CRS. Hahn Air operates only one route in its own right to meet, as mentioned above, the "airline" requirement. They have two weekly flights from DUS to LUX on a Cessna Citation, just like Flexflight.
Flexflight have been in the media in Germany because they initially planned to operate RBM-SZG and offerend flights on that route - without the knowledge of the airport. When an aviation enthusiast booked the flight and showed up at RBM airport, the airport refused to let the flight operate as a scheduled flight. So the passenger and the operator turned the flight to a charter flight on the spot . After that experience, the airline moved the flight to AGB. They don't really expect passengers as the route is quite useless, so it is quite a spectacle whenever an aviation buff throws some money at them and books a flight.