While I don't doubt the Mongol brutality, the death tolls just seem so much higher than similar events that an additional factor seemed plausible.
IIRC - the First World War (1914-1918) was the first big conflict that killed more people with bombs and guns than through hunger and disease.
Our ancestors would count disease deaths that happened during the war towards the total death toll.
Exporting hundreds of thousands or even millions of healthy young soldiers around the world with a dose of an infectious disease is a perfect recipe.
They last a long time and give diseases lots of time to spread in the close quarters of ships and barracks and then their homes.
Interestingly, Austrian-Hungarian soldiers were also transported to the front in cattle cars (8 Pferde oder 48 Mann = 8 horses or 48 men in a car) and were very jealous of the Germans whose military bureaucracy tried to provide normal passenger railway cars to their troops if possible. The difference was very obvious in the places where the two armies fought side by side, such as Galicia.
There may be other short small wars in which arms killed the majority of victims.