I'm not sure if that is true. These are the peak years for Austria, West Germany and the Netherlands from the table in Post 80.Matzen is in Austria, Schoonebeek is in the Netherlands. The North German fields are relatively minor in comparison with those. That really doesn't help Germany until Austria is annexed. IIRC the Matzen field is pretty deep too, so isn't readily accessible to the methods of the 1930s.
3,608,211 Long Tons - Austria 1955
7,801,994 Long Tons - West Germany 1967
2,357,400 Long Tons - Netherlands 1965