But was it being "propped up" two centuries ago?

Unpaid externalities is a fancy way of saying propped up.

Where’s my citation I asked for?

I’m not saying the parent is necessarily wrong, but given the recency of the industrial revolution and the length of time between which it “began” and when many of the most useful industrial applications for coal were discovered, I can’t take it at face value “centuries” is correct. It might be close to correct, it might be technically correct, but it’s not merely an pedantic point: his comment reads as hyperbole because of the vagueness of what centuries even means here. Even if it has been technically two centuries, century plural technically, that’s overstating his case. Overstating your case is a good way to get people to turn off to what you are saying if they perceive it that way.

It is very easy to look up the history of coal use and see that it had metallurgical applications in antiquity: https://en.wikipedia.org/wiki/Coal#History

A lot of the coal used in Swedish metal-working (up to relatively late, but I don't actually have dates, here, gut feel is 18050s) was charcoal, rather than mined coal. With quite advanced cooperatives[1] producing the charcoal for the mining/refining cooperatives[1].

[1] Somewhat loose translation of "kolare" and "bergslag", here.