The point is, a subsidy is when one entity possesses resources (money, equipment, etc.) and grants those resources to another party without expectation of a positive return on investment. Some argument can be made that a tax break is a subsidy, but I've often seen that framing used to mislead people into thinking that their tax dollars are actually being spent on the recipient of the tax break (e.g. Amazon in NYC). Trying to put speculative and indirect costs predicted to happen decades in the future under the label of subsidies crosses the line into intellectual dishonesty.

It's not hard to make good arguments for decarbonization of energy production, so using rhetoric with questionable intellectual honesty is often counterproductive.

Here's my reply to your comment below, HN is probably limiting a shared IP:

Your question is self-contradicting. "What if an entity doesn’t possess those resources, but gives them anyway" If an entity does not possess those resources, it cannot give them away. So the crux of the question is whether taking resources from another entity is a subsidy and the answer is no. When the Romans enslaved their neighbors and took their land, it'd be ridiculous to say that the Gauls, Carthaginians, etc. "subsidized" the Roman Republic and Empire. They did not help the Romans expand, in fact they resisted their expansion through force. Affecting climate through greenhouse gas emissions is much less direct than military conquest, but the dynamic is the same.

Good arguments can be made by pointing out that emissions are harmful and degrade our ability to live in our environments, even though carbon free energy production is more expensive. The consequence of people resorting to intellectual dishonesty to try and inflate the cost of fossil fuels make people think that proponents of climate change are naive to the economic reality that carbon free energy, especially renewables, are much more expensive to produce.