The article thus asks: if we lost those tools, and the knowledge/infra to make them, would we have enough readily available energy to practice and re-discover them, given that you need those tools to get to the remaining energy?
So even what we currently think of as "small" or "negligible" deposits might be more than enough for productive use within the now-smaller population.
What matters is volume and economics. The amount of man-hours per tonne of coal extracted by modern mines is far, far lower than during the early phases of industrialization. And that means that some coal mines, even ones that are easily mined, are just not economically competitive in modern times. But they're still there, the coal's still there, it's still easy to get at.