The title is a bit clickbaity and, as usual, makes a much stronger claim than the research suggests. Sure maybe it was the climate, or maybe it was just regression to the mean.

There were roughly 100x as many Europeans as Mongolians in the 13th century. It seems unlikely they could have ever conquered all of Europe with supply lines stretching back to Asia. All while maintaining a grip on 100 million subjugated non-Mongolians who probably weren’t keen to send and supply troops across the world.

Conquering they much territory is an extremely unusual event, it seems reasonable that the Mongols would just eventually run out of steam.