In the case of this discussion, you're looking at it backwards. The subject of fossil fuel subsidies was brought up in comparison to subsidies for renewables which in this case were already defined as specific government programs. If it weren't for that, I wouldn't be being such a stickler. However given that we are comparing renewables to fossil fuels we need to be sure that we are measuring the same thing and to include externalities when measuring fossil fuels and not when measuring renewables is to be not measuring the same thing. (I actually got confused when I first saw the IMF link so I know my concern isn't hypothetical.)
Yes, I get that the case can be made that cost of using fossil fuels is more that the sum of direct gifts by the government to fossil fuel companies (fucking duh!) but that can be expressed without implying that governments gave $5.2T in tax dollars directly to fossil fuel companies in 2017. Where confusion is possible, it's best to make distinctions and clarify what is meant.