You pay a grocer who pays a farmer who pays a fertiliser manufacturer who buys some natural gas. You wanting to eat has a reasonably direct impact on demand for fossil fuels, and therefore prices and profits. The oil company doesn't capture all the value, partly because they're not doing all the work, and partly because of competition.
If the oil companies went away tomorrow, the price of oil would skyrocket, because people want to eat, and new oil companies would form to meet the demand. There is no need for subsidy.
Positive externalities are things like "I financed construction of a road for my own commercial reasons, but other people can use it for free". Or "I wanted to renovate the rundown building I live in, but doing so made my neighbour happy too".
But to come back to the point, finally. The argument is that we know crypto mining has negative externalities (crypto miners don't have to reimburse people negatively impacted by climate change). The article argues that permissionless blockchains are doomed, ultimately, to fail anyway. So they will destroy value, by pointlessly wasting fuel AND by harming the climate (which doesn't factor into a miner's decision-making because it's an externality).