It may be the case that coal is cheaper if you ignore the capital cost of the plant (e.g. running an old plant beyond its design life) but including capital costs amortised over the lifetime of the plant coal is not even cheaper than natural gas.

My understanding is that a major source of electricity used for bitcoin mining in China was hydro power during the wet season (when there is a natural power surplus because the level in the dam must be reduced)

The reason it's the cheapest is that you're negotiating with somebody who is desperate and has no options left. They have to sell the energy to you for bitcoin production because there's literally nowhere else they can turn.

The more green energy proliferates and climate change action expands, the more of these plants will be reduced to this desperation. The worst energy production is the cheapest, so long as it's been outlawed and the plant has no other options.