It does make sense that Maine would have some. I know New Brunswick Canada has a HUGE deposit but much of it stretches out into the Atlantic Ocean. It had a few mines on land that actively mined the coal. I cannot remember if it was bituminous coal or anthracite, which is what surprised me about the coal find in Maine...it was anthracite and of "very high quality". But boy, 2800 feet is a long ways down.
BTW: They say a huge reserve of oil lies off the Maine coast but no one is allowed to drill for it. I can see where drilling past 3000 feet would have been difficult in 1921 technology when the coal was discovered, not to mention the VERY abrasive nature of coal taking its toll on the drill bit. But I bet if they could have drilled deeper, they would have struck some. If I remember right, the gas and oil wells of Wyoming are down 6000 feet. As with most things, its not that it is not there, but rather how worthwhile it is to obtain. The tar sands of Alberta have been known for a long time and is twice as big as the middle east reservoir, but the cost of converting it to usable oil required the price of crude to be high. As prices go up, more oil could be obtained from formerly non-worthwhile resources.