Personally I don't think the "perfect substitution" argument is that great in the first place (since it fails to take into account what it means for more supply to be on the market).
"if that mine doesn’t supply coal, then another will." smells of assuming that demand is fixed, use is insensitive to price and that substitution can't happen.
... BUT I also don't follow the logic behind this ruling.
You should read the (excellently named) No Country for Coal Gen report, which argued in detail that closing coal power plants was the economicly efficient thing to do but that various regulations meant they'd stay open long after they became net negatives costing tens of billions.
https://carbontracker.org/reports/no-country-for-coal-gen-be...
Population and per capita consumption have both increased for more than a century so the long term data is hard on the side of "demand relentlessly increases" (for major mineral demands and for energy supplies).
The logic here behind this ruling is that within the global thermal coal market it appears that demand is levelling and peaking, coal is on the nose, and thermal coal demand will start to fall within the decade.
That's based up such things as (say) the Standard & Poor (of SP100 fame) 2022 global coal forecast report.
What they do as finnancial players in the stock market, though, is largely orthogonal to the market reports from independent experts that they commission or purchase.
The entire S&P global mineral intelligence wing was aquired from a third party and was built up to reflect the ground truth of lease acquisition, exploration, capital development, etc in the mineral resources domain.