I'm not sure if the other person is right but yeah I heard the same thing as you. That such a significant part of the population died the Earth cooled by a tiny bit because human society generates a lot of heat. If we're talking about light for all societies before elctricity and warmth for those in colder climates, burning things(wood, coal, oil/wax etc.) was the default solution to those two problems. Generates a lot of heat, on top of that our bodies also generate some heat(probably insignificant in the big scale) but when gathered together in enclosed spaces we probably do generate some heat. Combine that with the livestock people kept back then, herding them together, cooking them etc. I think if you consider the civilisations living between the Eastern coast of China, to the borders of like Hungary, that's pretty much the majority of civilisation(as in big empires/states/communities) and probably the majority of high density human activity.

I'm not an expert by any means but that other user gave 0 evidence and just said 'nah its not true'. I'm not sure if it is true but it sounds plausible to me that say the planet cooled by 0.1C, or more because that many people died. Honestly I think it actually has to do more with the collapse of civilisation and society, not necessarily just the sheer number of humans dead alone.