The 200 year supply you often hear mentioned is total resources in the country both anthracite and soft coal. Of course soft coal makes up the bulk, anthracite is just a blip.
Just to put this chart into perspective the very top of this pyramid in dark brown at active mines represents about 2 decades. The second part is the important one as it's known reserves that can be mined, e.g. they are excluding things like if the coal is under a town or not feasible to extract. That is where the two centuries come from. We burn about 1 billion tons per year but they take into account increased usage. It's actually closer to about 175 years.
As far as anthracite goes I'm not aware of specific information however I have sen numbers like 6 billion to 10 billion tons but I believe that includes everything. If they were able to mine all of it at current consumption rates that is a few thousand years give or take a thousand. The point of course is that it's lot but a lot of it is inaccessible. Whatever the case I wouldn't go worrying about it.