Looking at
these tables here, its obvious that at least regarding brown coal Natzi Germany had just "tapped" the East Elbian brown coal reserves compared to what the GDR get out of the soil in the post-war era alone.
Also the bohemian and moravian lignite fields were and stayed "underused" compared to the postwar era.
Why they didn't exploited them ... atm not much of a clue, but that they didn't do it IOTL does NOT mean they would not have could.
But ... the above linked site also shows, that the total production from 1938 to 1944 exceeded the total consumption of the same time (including exports).
It seems more that it was less a question of the coal at hand than the facilities/synthetic-oil-products plants to process the coal.