I had a family member that owned property in WV. They looked into the mineral rights for the property. The seam below the property would be difficult to mine due to a near by river. To access the seam the water would need to be diverted. With a high enough price for coal then these sorts of seams become mineable.
Actually, the article mentions that purchasers should consider buying the harder to mine coal deposits first, precisely for that reason:
"Harstad thinks that his coalition should lease or purchase the hardest coal to access: In other words, the coal that’s most expensive to mine. Then, as demand for coal grows and the price rises—and mining companies start to look more more difficult reserves—they’ll find that coal is already locked up."
The craziest aspect of it, is the assumption that you could pull a meaningful fraction of known mines off-line to accomplish the end goal of seizing up the coal market (essentially crashing it, such that it can't recover).
Here's what would actually happen: Gates & Co. buy up 10% of mines (at a huge $ cost), the price of coal rises say 50%, increasing the profitability of the other 90% of the industry. The other 90% then invests those gains into more output, more exploration, and they bring on-line what were formerly less profitable deposits (Gates & Co. will not be able to buy all of those, the planet is a big place, and there's an extraordinary amount of coal).
You would have to buy a truly massive number of mines, in an extremely short amount of time, to accomplish the plan. The moment you start trying to buy up mines, the price of each of them will rise, so you'd have to do it impossibly fast.
There's no scenario you can walk through in which it's a feasible plan. Not even close.
So yes, entrenched actors (i.e. coal companies) will react to a rise in the price of coal by mining for more coal. However, many other actors will be incentivized to innovate and develop new sources of energy.
More expensive coal can help speed the process, I certainly agree with that. The question is, can you buy enough coal mines around the world to crash the coal industry in that respect, and out-pace the amount of new coal supply other miners can bring to the market seeking higher profits. I seriously doubt it, I'd argue there's far too much coal.
Politically you'd have to basically use the Obama Admin tactics, globally, to hit the coal industry, while also removing as many mines as possible. It's an incredibly expensive, risky proposition (and good luck getting everyone to sign on to it) - when we'd be far better off continuing to improve alt energy, and building nuclear plants. China for example isn't going to chop their own legs off to placate climate concerns, they're going to keep using coal until they can gradually diversify to other sources like nuclear, wind and solar.